I don't know that it is as big a problem as all that. Two Nth level spells are going to be more powerful than one Nth level spell upcast to 2N in most cases. If the feature doesn't allow you to actually cast the higher level spell and only applies to updating it's fine. Even if it allows you to cast higher level spells you still need to be capable of casting that spell level in order to do so (it doesn't let you exceed spells known/prepared limitations).
Though if the latter you might specify that spells above 5th level normally can't be cast in those higher level spell slots, since those are normally restricted per day more than 5th and below.