Exactly, the rowid of the relationship... Oh damn...
Happens not that often I think. I mean multiple performances of one artist at the same event are rare. And then updates to them are even more rare. Normal the schedule stays the same.
You are right, in most cases I can make some smart matching and it will work. But some cases are impossible to match.
You expectation is correct. In the rare rare rare case when I can't match, it will lead to loss of data for the user. I think it happens too rare to burden the MB team with such a big change. 
I will then leave the choice to the user: Loose data while update (I mean they can screenshot it and fix it after the update) or do not update this event at all.