I'd let it swap out a spell of 5th level or lower on a permanent basis. I don't think it's that powerful to allow, a sorcerer is already at a spell caster disadvantage at higher levels due to getting the next level of spells 1 level later, and I've never seen that swaping a spell is that powerful. However, since you're casting it as a 7th level spell, I'd limit it as if that was your max spell level (but I'd let wish swap it out regardless of level). And if a sorcerer abuses it, 300 XP will still add up.