It'd be much better if you just told us what flights you're talking about rather than being vague and cagey. We're not going to track you down and steal your identity just from knowing where you're flying to!

I would think that as OP is considering replacing an AA flight with a BA flight, there's almost certainly going to be a AA code for that BA flight.

After all it can only be one of 3 situations:
AA and BA flight on the same/similar route - almost certainly have codeshare
AA flight that BA don't operate (intra US) - no use to OP because he's talking about a BA flight
BA flight that AA don't operate (European) - I can't see why OP would book onto this as a replacement for his AA flight.

The other situation would be replacing a single AA flight with a connecting flight - e.g. JFK-MAD replaced with JFK-LHR-MAD.
These would be fine as the connecting flights can usually be booked with an AA code when connecting from a Longhaul from the US.