DEFINITELY current legs becoming hind legs, with the body stretching out and a new torso and front paws forming out of your belly
the taur front paws aren't legs, they're arms. i feel like it would be weird for your legs to become (quadruped) arms and then more body to appear out of your butt. like it feels like doing more steps than necessary to reach the same end result?
like you'd need to change legs into arms and add new legs and hips, while my way is only adding a new chest and arms in the middle.
furthermore i think that the feeling is more fun when what used to be your legs (what you can still "feel" as your legs) are suddenly very far away behind your. and where you used to stand on your legs (directly below your torso) you now stand on new, foreign feeling, front paws.
and you have full control over these front paws, but the neural pathways are completely new, raising your new front paw would feel like raising your arm (but a different arm from the ones you already had) but you'd sense it in the place where your legs used to be... it would feel so weird but good ^^;