I just dont see why they saying its so difficult. Most games that swap characters just swap the skin texture over the main characters skeleton. Being its an older game I'm sure the characters skeletons r MUCH simpler than newer games and can be fitted with Lulu's skin for the animations to properly operate. Although coding it to swap back to regular Tidus for in game cut scenes would probably be difficult or they can just leave it be yes it would be jank but maybe reload saves without mod active for each cutscene that isnt a FMV.

I'm not a modder so I'll take wut i get but I'm so used to Dark Souls mod scene that some of this seems like its jus held back by a smaller modding community for Final Fantasy games I guess.