I have a DL flight coming up in a week that I just checked online and noticed that my FF number wasn't the one I wanted to use and I got to asking myself: when is the last possible moment to switch FF numbers? Before check in? Before boarding? Could you potentially swipe your BP then (by asking nicely) talk to the GA and have them switch it? Can you switch mid-trip? Has anyone tried?

Losing upgrades/benefits/etc aside, when is the last possible moment to switch FF numbers on a DL flight?