Although I'm a UK resident, the benefits of AAdvantage suit me a lot better than Avios. However, as AA only flies in one direction out of here, I often find myself travelling with BA. I've begun to think that I might as well open an Avios account and start collecting miles from shopping and hotels and whatnot so that I have the chance of upgrading with BA once in a blue moon. This thread has taught me that you can put your Avios number in your record and upgrade online if and when available, and then switch to your AAdvantage number. The disadvantage of this would be that I won't be able to preselect my seats (which will matter if I don't get my upgrade). I guess one way of mitigating this risk would be to set up EF alerts for my preferred seats and then forget about online upgrades and make the switch if they start disappearing.

However, if I open an Avios account, will BA's computer start trying to replace my AAdvantage number with my Avios number at inconvenient times without asking me? Even though I have a BA.com profile, I still need to enter my AAdvantage number manually every time I make a new booking.

Also, if you upgrade, do you end up in a booking class where you won't automatically get AA credits and if yes, does it take a lot of explanation to get retroactive credit?