Even as a PM now, I still get great service from most phone agents, though calls take longer to connect (2-5 mins usually) than when I was a DM (no wait at all). This makes sense given what Widgets said.

Also, I had a completely different experience in a very similar scenario as the OP last year. There was a schedule change on a flight I had upgraded with a GUC and my travel agent rebooked me on my original routing but a day later. When I called Delta, they rebooked me on a different routing on my original day of departure and maintained my upgraded J class of service even though the new TATL flight was AF-operated (and I didn’t have a Y/B/M ticket).