Most of it? P3P didn't have animated cutscenes and, while it added a lot of extra characters, it mostly was replacing NPCs with romancable party members (as you couldn't befriend your male party members in vanilla P3). For P5, you'd have to write wholly-original new arcs for new characters. The P5 menus have Joker moving around throughout them, which wasn't a thing for P3P.
The FeMC could be made to be less unique then the P3 one (the FeMC in P3 is much more emotional then P3 male MC). They also could, if they wanted, make her have a metric craptonne of female-female romantic relationships if they didn't want to have to rewrite/replace the male ones. But they'd still have to rewrite the story in somewhat major ways, because the game is still about rebelling against Japanese norms, and Japanese norms are heavily gendered. Moreover, if they were to have it be a somewhat shallow swap, then people would probably be less excited about it.
Like, I still think it may happen, but it would be an insanely big undertaking.