Originally Posted by
fone
Personally, I've had issues with this. Checked in with A3 number as required the free checked bag, proceeded to lounge etc. Got transfer desk at SIN to change the FFP number (to UA, as I have sufficient for A3 *G) and got a new boarding pass, boarded with the new boarding pass. In the end, still credited to the original FFP, which is A3.
When the boarding pass matches up to the FFP you want to credit to, you have a reasonably strong case for retro-crediting. I normally opt for deletion and a blank FFP field on boarding pass, which is IMO even safer unless the first FFP is that of the operating carrier.