changing FF number at the gate
 

Asked a GA to change my FF number before boarding - she tried but couldn't. Tried the agent at the Delta "help centre" in the terminal. And at the Skyclub.

All of them told me that the FF number could not be changed "after my travel had begun". DL stock, DL metal, third leg of TATL 8 or so.

I've had no problems doing so before - any ideas why this might be happening ?

Your not supposed to be able to change it once travel begins. It's been that way for awhile, I'm guessing the computers now are programmed to block an agent from doing so. They're probably wanting to prevent Medallions from getting their benefits and then crediting to another airline FFP and getting status on them also. ie a number of Medallions do this and pickup AS status once they've finished renewing their Medallion status.

I know United were trying to do the same, not sure if they did or not.

AFAIK, there's no regulation that mandates allocating miles to the card that gives you benefits - I guess it is individual company policy.

I never understood why this point was an issue - I would assume it all balances out - for example.I would fly AF, credit miles to AZ, and use DL status for priority/lounges - and someone else would fly DL, credit AF and use AZ status, which would even out across all airlines.