I had a couple thoughts, let me know if anyone has tried these or know what will happen if I try them:
1) Wait until I get to the gate, and if space is still available, try to get the GA to pull 500 milers from my DM account and still credit my MP account.
2) Switch before the flight to my DM number, then once my upgrade clears, call and switch back to my united MP number. Then when I check in, I can let the GA sort it out like in #1.
*Possible problem, the first leg of my flight is US code share flight on UA metal. Since I am checking in with UA, what will the UA agent do with my upgraded flight? Do they even have the power to pull the 500 milers? How would this work if I was simply crediting the miles to US DM?