The actual sprite movement (ie; animations, not movement along the field) is governed by the .a files. So...it's not a matter of swapping the HRCs, but *every* .a file....which will probably result in more problems than it's worth, seeing as the vast majority of the sprites (including cloud) are 21 bone sprites; Meaning, if you fiddle with the animations that Cloud uses, there's a good chance you'll end up having problems with most of the other sprites you come in contact with.
It seems to me that the field files tell which animation to use, as well as how the sprite is moved along the background...So, if there's been any luck decoding that part of the game, the thing to do would be to replace Cloud's scripted animations with the corresponding ones for Sephiroth.
Also, another possibility I just thought of...
In fact, aside from some minor model editing, it's really the easiest way;
Eliminate the two extra Bones for Sephiroth's hair, and either enjoy the haircut, or bust open Ultima and manually add in the hair to the head model. It won't move if you do that, but it's feasible.
I'm headed to sleep now, if anyone would like me to go ahead and edit the actual .p and HRC files tomorrow, let me know. I'm not sure how to go about making a patch for them, though...