Changing dialogue is entirely optional, and the OP did simply state changing the appearance of Cait Sith to be Sephiroth, not a 100% character role change.
but, for a 100% character role change, changing dialogue is easy enough with loveless and other tools that were out for a number of years.
The way I suggested to do a physical change was easily doable at any point for about 5-6 years.
Although, care to tell me how exactly it ruins the game to have the evil Cait Sith physically be someone else, but retain the original dialogue?
It would take a while for one person to change every single line of dialogue Cait has to be new material as if Sephy said it, and would more than quadrupole the time spent on the mod.
Without changing dialogue, I personally fully replaced Cait Sith with Sephiroth in battle, (giving him 13 different weapons) the field, (all of his field models) the menu icons, the field animations, and I gave him a new introduction text for when he joined the team.
and it took me about 20-25 mins to do that.
the dialogue is just an added touch and would take several hours more than likely to change every instance he says something, and unless its going to be a published mod, I don't see how it creates storyline problems just because he has the same dialogue.
Its an aesthetic change more than anything, like wanting to use Blank in FF9 instead of Zidane.
okay, now i see where i went wrong, the guys i'm talking about weren't replacing him with another existing character with existing animations, they were creating an entirely new character with a completely new backstory, changing alot of how the plot went, new animations in battle and field, etc.
they were calling him Skoll, if my memory serves correctly.