Bit of an odd question, is it possible for an down-tree or root member of a clade to merge with an up-tree cocladist? Essentially, willingly subsuming themselves into an up-tree member of their clade?
Spoiler level: none
Dear, Also, The Tree That Was Felled:
Yes. Sort of. We call this 'bubbling up', which is when an individuated fork merges down and then the down-tree instance assumes their identity. The issue, however, is that when merging, the down-tree instance has the ability to selectively merge memories, while they cannot release their own memories, except in the instance where there are conflicting memories, wherein one can choose the up-tree instance's memories — this usually means a reinforcement to the point where the down-tree instance's memories in those cases feel more like a whimsical imagining rather than quite real.
See also: Mitzvot - "She Who Haunts The Storm"