Accrue BA but use AAPlat status?
 

Maybe I'm not understanding OW status. I'm AAPlat for 2017 but moved to UK and need to switch to BA because there's no way I'd fly four AA flights per year, they're all BA (crap earning rate for economy). However, I think I'm understanding that I can only use my OW Sapphire if I add the FFP with which the OW status is earned, correct? So if I want to change to BA I basically have to go back to nothing and start over and throw away my Sapphire? Or can I fly BA FFP and show the AAPlat card for those benefits?

There is no longer a requirement to fly any AA sectors to attain status, but if you wish to switch to BA, you can use Sapphire benefits

If you want to get benefits such as seat selection without charge, you will need to put the AA number in the booking and then swap it to the BA one at the airport

I did that once, and it still went to American so I had to have them remove the miles/mqm so that BA could give them to me.

If you want to switch to BA and get benefits, that is pretty much what you will need to do

Alternatively, given that there is no longer any requirement to fly AA sectors , perhaps stick with AA

That is a option, but AA only honors full-fare economy with 1:1 EQMs, so I'd never fly enough this year to regain status. However, all of my 2-6k flights with BA accrue the same 70 points, so it's only 9 legs to regain Sapphire (1:1@even 6k each would only be AAplat, and many of my legs aren't near 6k).

On some GDS it is possible, but finding an agent that knows the difference between the FQTV and FQTS fields can be difficult.

http://maphappy.org/2014/02/how-to-jump-frequent-flyer-programs-seamlessly/