You face the additional problem that if all that AC has issued is a waiver and has not cancelled or delayed the flight, that QR may refuse to act.

You are then caught in a no man's land until the day of travel (not necessarily 24 hours, depending on how QR chooses to look at it). At that point, you then get stuck on the availability front as others who were booked on UA/AC have already taken care of their changes.