So question for the OP. All that you were changing was a date on an existing ticket, and now you're being asked for the additional YQ amount?
Or did you make a change that would have triggered an award reissue (e.g. change origin/destination or carrier)?
If it's the latter case, then what you're really doing is issuing a new award, and in that case, you really should be charged the new YQ amounts.
If it's the former, I would be upset as well, as the "price" of the ticket was changed out from under me. I would be curious if AA is allowing customers in this specific situation to switch carriers without charging a penalty. I would also write to AA to see what they are offering to do. The tickets should be valid for the origin, destination, carrier and class booked. AA really should honor that, regardless of new agreement with BA.