I've had pretty good luck getting airport agents to "SDC" me onto flights when my original fare class wasn't open (usually P). Never had this lucky with economy fares, but with business/first they seem to be more accommodating.

Via phone I've had 0% success, but when I show up to the airport, explain that I need to get on an earlier flight, they'll usually say "oh your fare code isn't available" then I gently/politely ask "is there anything you can do" a few minutes later I end up with a BP and rebooked into Z/D/A.