You are supposed to be ahead of anyone at the gate who is requesting a complimentary medallion upgrade. You will be behind any DMs using an upgrade cert, or PMs using a cert on a higher fare.

The big caveat is the supposed to be part, since more often than not the upgrade doesn't get coded properly or passed to the airport waitlist correctly to give you higher priority. I suggest checking in online and closely watching the upgrade list. If you see yourself lower down then call and say that you're using an upgrade cert and ask it be applied for the airport upgrade list. You should be at or near the top.