Complicated situation. GPUs waitlisted for friends, who subsequently changed the flight to a non upgradeable fare.

Called MP as directed by OP. Reached agent fairly quickly (say "agent" at the MP menu). First agent took over 30 mins and still couldn't figure out how to do it - I ended up being cut off. Called back, got a competent agent, who recredited the GPUs in real time!

The only thing that bothered me was she needed the date of the original flight. (I only found by digging up my friend's email).