>>530027483
This isn't like previous SMT/Persona games in that regard.
In older Persona games, you would be required to keep a track of registering new personas, fusing them, and keeping them around the MCs current level because lower level personas fall off a roof in terms of power, stats, and also they just do less damage if they are going against much higher level enemies, and also MCs stats are entirely linked to what his current Persona offers him

In this game, MC gains all the stats of all 3 personas equipped, and there is a rank and rarity system alongside the usual level. Leanne's stats at level 35 will outweigh or match higher rank and level personas unless they are also the same rarity as her, and because you also get stats from other persona equipped, her damage won't fall off nearly as hard so equipping her for her strong Psio attack isn't immediately awful.

You should eventually replace them with higher ranked diamond personas when possible, but it's not a neccessity and you can wait until you have better options/more resources.