That, and perhaps a significant chance for the ritual to fail and kill the apprentice in a spectacular and horrific fashion?
I think that might be an interesting way to rebalance the grand convocation spells as well. Instead of consuming the warlock outright, give it a chance to fail and spawn some hostile monstrosity? Unless you have to have a creature for the transformation to work (I haven't really poked around with dfhack, not sure how that's implemented at the moment)? As another alternative, if apprentices were a thing, you could make the grand convocation spells consume them instead. I don't know, maybe people are happy with the way it works now, I just can't see my warlocks willingly sacrificing themselves for the spell effects, it seems too altruistic...
Edit:
Oh! Maybe there is a way to limit the number of apprentices to 1 per existing full warlock, or to just 1 total, like is the current situation with the overlord? That plus a time progression of two years and a chance for a failed initiation seems balanced to me. If your apprentice kicks the bucket in their invitation you'd have start over the time progression, you wouldn't just have an unlimited pool of other apprentices to redo the ritual with.