Blog goes blank page when rename theme?

Maybe it's best to ask at wordpress mu forum for this, but since I'm here, may I ask?

I have few users that their blogs go blank page since I renamed themes, or remove themes, or upgrade themes. The thing is I don't want to go into each user backend or options to fix it, it will be too much work.

Anyone has a better idea on how to quick fix this? Of course if a user log back into their account, switch to a working theme, and their blog will be OK as if nothing has happened. I want to do this so they don't have to do so. Thanks...