From my lived experiences, I think that TF "victims" would eventually forget what it feels like to be human. They may still desire to go back, but I think the longer they spend in their new forms, the less they'll know what it entails to go back.
I mostly say that because of all the changes I've gone through. Whether they voluntary or not, good or bad, I don't really remember what my life was like before they happened. I don't remember what my body felt like before I transitioned. Nor do I remember what it was like before I got disabled (excluding the disability I've had basically since birth, of course).
They're just things I live with. Do I wish some of these things would go away? Of course! But I do not know what that would actually feel like even if I have experienced it before.
Basically I just love some loss of humanity in transformation x3
So you're thinking to yourself, "Hey, you know memory recall is an active process" and then you're considering Neuroplasticity in the human brain, and you might think this kind of sensory-linked memory is immutable. Turns out, naw it isn't and the various hypotheses about why are pretty cool. Memory of all types is pretty fragile and prone to not only errors but also revisions during recall, one of the famous studies about adults who witnessed 9/11 is that the resolution of the memory got denser the further away the memory got (meaning more details "recalled") Hirst, et. al., 2010. This speaks to an interesting phenomena about human memory where the brain actively encodes and decodes memories during the recall process, I like to imagine our modeling of memory as unspecific: "We have the Zipcode, but not the Address". Memory Manipulation in humans is extremely variable with some having exceptional recall, resistance to re-encoding errors, or "super memory"Hyperthymesia, via Wikipedia. Out the other end of many of our studies about memory is that it's pretty fragile and the importance of documenting independent of our lived experiences is actually really quite high, and if you're unlucky enough to have ADHD like me you make errors in recall constantly.
The idea of what makes "you" stand out as "you" in particular is an interesting question physiologically speaking, so provided you're not going all-in on a more mythological / spiritual metaphysic, you can model out some quirks to make your Transformation fiction look interesting. It doesn't even have to be all that smart either, the brain's kinda squishy and molds in unexpected ways if you look at our case history!
The implications for post-human neurology in any conventional transformation series is staggering to consider, and in many ways the shape, density, structure, and chemistry of the human brain would not be preserved. This could range from mildly disquieting alterations of sensory experiences (since the brain actively processes memory Wheater, et. al., 2000) into full blown disordered recall potentially presenting with synesthetic symptoms (that's when 2 or more senses activate to stimulus, like flashes of colour when reading #'s), dyskinesia (involuntary jerking or movement of the limbs), or other clinically significant phenomena (potentially including seizures Das, Luczak, 2022 during memory recall). In many ways one would be lucky to have memory to speak of in the first place, as odds are good the reshaping of the brain would break or alter most of your Cerebral Cortex's layout and our understanding of human memory depends upon that structure due to architecture-specific findings. Most people know the study of HM who famously had his Hippocampus removed to control seizures, and that totally ended his capacity to form new memories (the Hippocampus is part of that encode/decode function of recall, it is inhibited in PTSD activation). What people don't often know is that Traumatic Brain Injury not only tends to impair your current memories, but can impair future memories too, making rates of both retrograde and anterograde (new and old memory recall) amnesia higher.
So when writing a Transformation story, we have to ask ourselves a few questions about the model of self and by extension what the Spirit of the Law for post-TF memory should be. Your brain is what makes you a person in specificity, with hypothetical models of human self dependent upon distributed and specialized processing and the 3-D structure of the brain. I have to bring her up because she's an Arizona politician and case study, but former House Representative Gabrielle Giffords was shot in the head at point blank range by a 9mm Glock during a mass shooting at one of her events (a grocery store about 2 miles from my home at the time), and her personality is mostly the same, albeit with more impairment and delay. What makes this interesting as a case study is that Gifford's injury to the left lobe impaired her language capabilities (and probably math too), but her undamaged right lobe mostly means her core personality characteristics are unaltered. She's incredibly lucky, but is another case example of how specificity of function in the brain matters; a lot of people would agree that left-hemisphere dominant thinkers are able to compartmentalize emotions better than right-hemisphere dominant thinkers as a general rule. In yonder past we used to have arguments about possible sexual dimorphism with the interweaving structure of the Corpus Coloseum, but these prior statements about sex-linked differences in structure were debated even in the late 90's, and are at best inconclusive Bishop, Wahelsten, 1997. This makes it hard to say, in specificity, what would change about your personality were your brain to change shape.
So what can we potentially envision? Well, the human brain's top structure, the Cerebral Cortex, has four main lobes: Occipital, Temporal, Parietal, Frontal. Each of these lopes have specific functions and if you want you can look 'em up, but I'd consider that if your noggin changes "you" should probably change with it. In Animorphs, the characters are (assuming my recall is correct) supposed to have some kind of advanced technology that preserves the neural map or whatever of the character's to keep them from experiencing significant changes in their Cognitive capabilities when turning into mundane animals from Earth. This technology's limitations has important ramifications, such as the kid who becomes a bird forever seemingly loses a lot of those capabilities and it's kind of hard to tease about just how much they remain themselves in that form. Going from well established fiction into my own, I use the concept of the Neural Map too be a structure-specific firing pattern of neurons which can be encoded and decoded by advanced technology in real-time, eventually allowing for its literal "export" from one Cognitive Media to another. I don't pretend to know how it works, but I assume Electromagnetism is involved in the process of packaging and moving the consciousness. If Consciousness is an epiphenomena (a phenomena arising from other phenomena), there's an implication that a replication of those smaller phenomena should bring about that epiphenomena. At least, that's the hypothesis I operate on and model, and the density of the equation here is outside of my knowledge.
So what happens if you do a hokey scifi "brain swap" using this methodology? Honestly, I don't know, advanced technology 'demagnetizing' the brain at synapse-specific resolution might literally kill the brain entirely, we're not totally sure how that'd work. But consider that if memory is an active recall process, memory alteration is possible during recall, and the brain uses its various processing zones (sensory and otherwise) to perform recall, we can probably do the same "backwards" on purpose to encode new memories and new connections. There probably should be a degree of error. Most people differ from brain-to-brain on specific densities of Grey Matter (you don't want mine, my left hemisphere frontal lobe is a lot less dense than yours probably thanks to about 20+ years of treatment resistant depression and recent narcolepsy), so your mileage may vary on what you can do if the brain is being 're-encoded' to emulate a novel (to it) Consciousness. In my still-in-progress Science Fiction the Zevash Codex, an advanced and anomalous material becomes part of a planet's biosphere, resulting in 'psuedo-psychic' flora and fauna that use electromagnetic waves in various parts of their survival strategies (signaling mates, proton pumps, etc). The planet's big sapient native species, the Zevashi Roach, has used agriculture techniques to make these spire-and-moss lines that resemble power-lines, allowing for long distance communication and for privileged members, the ability to eject their consciousness and respawn with a clone egg; very high preservation of their brain data too.
We can consider that with Transformation from one body form into another, the complications would be perhaps less exaggerated than transfer from brain to brain. Since the brain is actively changing form, you may experience neuroplastic preservation for most of the essentials. You might remember things like your favorite foods, perhaps recognize certain rhythms and other core memories with significant strength and reinforcement (music recall in particular, very good at surviving brain damage as we know with Alzheimer's/Dementia). For Synesthetic people, odds are good your synesthesia won't change all that much and you might even have better skills related to that synesthesia as a result of changes in how your brain folds (those wrinkles are important) or where the grey matter ends up going. That said, I would also consider as a writer to make certain things about your character either paranormal or extranormal in regards to their memory recall, or commit to making their pre-transformation life a vague low-fidelity 'snapshot' that develops with mutations over time. They might be prone to inserting new memories where old one's have a basic 'echo' of familiarity, struggle with maybe recognizing their former human face and seeing it in historic documents like videos and photos. You can even play on a reverse-dysphoria if you want to be cruel to your OC's and give them a psychological dissonance where familiar faces make them confused, emotional, or otherwise distressed. I really like the idea of a paralingual self that is more of a pattern of behavior and emotion, with the intellectual elements of self being largely semantic memory.
So our last topic for discussion is Semantic and Episodic Memory, which I should've covered first probably. Semantic memory is kind of your 'facts' memory; E.G. "the capitol of Arizona is Phoenix" kind of stuff. Episodic Memory is what it says on the tin: that recall of things that happen during your life "when I learned about Phoenix, Arizona, I was huge into mythology and now I always think of the bird". Episodic Memory as you might've guessed is more distributed through the brain and is more prone to disturbing by damage to our recall/encode abilities; PTSD in particular often causes this 'stripping' of Episodic markers, creating a dissonant recall with effective narrative gaps between memories. As someone who experiences distortions in Episodic Memory (I have something rhyming with PTSD starting age 4+), it feels natural to me that a Transformed character might have trouble remembering things about themselves in that Episodic manner. REP, this account's alias, has that exact high-concept down (spoilers REP is post-TF surprise) and is some kind of synthetic body hosting a repackaged consciousness (both change of brain and brain to brain). Who they feel comfortable around, how they act, and what they know is largely based on Semantic Memory and these more vague behavioral quirks, plus that sensation of familiarity which comes with people they used to know. As a result, REP is blissfully unaware of who they were or that there was a "who" in the first place, and spoilers again, it's only implied by the evidence of the circumstances of their production.
So that's my incoherent and rambling thoughts about Transformation and Memory; accounting for changes in the Brain can mean many things depending on the specifics of what parts of the brain you think are most important to "you". People familiar with Lobotomy literature would argue that the Frontal Lobe is the most "you" it gets, but as you read above, there's a lot of brain involved in the experience of "you", and all of it is pretty important. Shout-outs to SOMA, which in particular discusses the idea of Continuity of Consciousness, the ever-critical idea of self as our recall of events and not merely the documentation of ourselves during events. It's pretty interesting but only one of many great examples of memory play in fiction, you can find a lot of it also in the (rightfully) maligned Bioshock Infinite, where the protagonist is actively being lobotomized with every step they take as their concept and recall of self is fucked around with like it's silly-putty. I think the first Bioshock does the high concept of the fake-self a lot better, "would you kindly" evoking non-player obedience where the player at first thinks they have more control over the narrative. Instead, the player is seemingly the Bioshock protagonist's active self, writing new memories over the fake ones implanted in them when they were put out in the world as a sleeper agent for Atlas. Oh there's a new Outlast game that also technically covers this but eh, I don't think it's very good. Play Disco Elysium instead for retrograde amnesia and the rediscovered self through interaction with one's personal history and living world, it is a genuine treat (though the schizophrenia or DID your protagonist has can make the game a little less approachable).
So, get out there with your TF writing and play around a little with your character's sense of self, their memories, and other brain-bits!
Heh. Memory play. That's probably one of my kinks.
