Originally Posted by Dr. HFH

I'm going to give it a definite maybe. In theory, yes, you should be able to change your frequent flyer number for each flight. Usually you can do it either at the gate or in the elite lounge (if you have status to gain admission). However, there are reports on FT of trying to do it, and the agent says that the number was changed as requested, but then getting NO credit for a flight because each program claims that the flight was already credited to another program, regardless of what the boarding pass says.