I don't know the exact answer, but if I had to guess, I would say this is no problem at all. I believe the 24 hour rule comes from the fact that Star Alliance Partners have 24 hours to reject an award booking, which would include changes. This doesn't mean they will, and it doesn't mean UA won't let you book one, but it means it could happen.
A cancellation on the other hand would process with no issue for both sides.