I have the same question.

On an AA ticket: TPA-MIA on AA, MIA-LHR on BA, then LHR-IST on BA. Return is the reverse.

I called AA and asked about upgrade from WT to WT+ on the MIA-LHR segment, over the pond. Referred to two different individuals, on to a supervisor, who tried to work it for me but concluded it was impossible to do on AA software.

My question to this forum then becomes:

Once I am ticketed through AA, can I go onto the BA site and do a paid upgrade of my BA TATL segment? Or might I receive a POUG offer?

Thanks.