Originally Posted by
sebbelcher
The only thing I think you'd lose is upgrade opportunity as I suspect you'd need to have your BA FFN in the system at the point of boarding. But who knows... it might still be possible to change it on the in-flight Wifi. We will have to experiment.
Often when I've gotten upgraded as AA Plat Pro on BA, its been at the check-in for a subsequent flight rather than at a gate (As my pattern is to fly in from the US but then head out for a hotel stay and back in via the First Wing at LHR T5). Other times (like just this Tuesday gone), I noticed I was upgraded when I went to check in online the 24hrs before. So I do wonder if its possible the upgrades/seat assignments would stick at that point if you then then asked at the check-in desk for the FF to be changed over to BA just before heading through to departures.