I think a lot of the pedestrians and background characters can be fudged in some way by using the heads from P5's background models and then making/editing a basic Yasogami High uniform for each gender. Those arent really that bad imo. For more unique character models youd have to look into getting...

Another problem would be a bit of an artstyle disparity as even some main characters like Dojima and most of the social links do not have HD models from the dancing games so those would either have to be made from scratch (which would be difficult to replicate the art style with it looking seamless,...