Persona 3 and 4 really changed it up. Persona 1 and 2EP (haven’t played IS yet) are much more like bog-standard SMT, with a few tweaks to the system.
Persona 1 and 2 had everyone able to change Personas. They also featured demon negotiation (a particularly interesting version of it, with 4 different emotional states for the various demons).
Persona 3 and 4 both have only the MC able to change personas. There’s a story explanation, but I think the real reason is the Social Link mechanic heavily affects making Personas, and giving each party member a Social Link system would be too complex. I kind of miss that, but ultimately P3 is a much better game with the S.Link system in place.
Similarly, negotiation was ditched; the high quality of the writing in the S.Links, and the rest of the story in general, make the interactions through negotiation look very shallow in comparison. And to be honest, they always have been shallow. There are just so many that it would be hard for it to be deep and actually make the demons their own characters. Especially when you fight dozens of copies of the demon, and they all have exactly the same dialog. So they changed the story a bit; instead of ‘demons’ like in standard SMT (and indeed, like in Persona 1/2IS/2EP) they’re ‘shadows’ – essentially a mindless rampaging horde.
Basically, if you’re expecting a standard SMT game, you won’t find it. P3 and P4 are very different. But that allows them to be their own games; they feel much less like “SMT, only not as good”. P3 and P4 are much more like how SMT would work if it were much more story heavy.