The Sims that you have played or your Sims have interacted with won't be turned into Spellcasters, according to the SimGurus. Only unplayed townies will show up in the Realm and only when you go there with your played Sim.
So if you never visit the Realm, there's not going to be any magic in your save.
If you visit the Realm with a Sim, only Sims that aren't played by you or on your Sims' relationship list will show up and be Spellcasters.
Also, even if Sims turn into Spellcasters, you won't see them do anything magic outside of the magic realm other than fly off on a broom.
