I think you are going to be out of luck

Since you are booking through AA on AA fares, you are not going to be able to purchase BA WT+ without cancelling the exisiting booking ( assuming it is cancellable ) , taking a hit on cancellation penalties and then rebooking afresh.

If the through fare in WT+ is only EUR200 more would it perhaps not be better value just to book WT+ the whole way and save the miles for another day?

Dave