2nd time in recent history that PAX has been forced to pay fee when AA switched to another airline (other thread is about a voluntary switch due to overbooking). IMO, airlines don't have their ducks in a row about this matter -- OF COURSE, the PAX shouldn't have to pay...the full cost of the air travel should be switched over including luggage (it's not like it's a buy-on-board meal) OR AA should refund the fee already paid to it to PAX when switching over tix (what a bureaucratic mess that would be though). Please complain to DOT. DOT will eventually hopefully post some policy if/when they receive enough complaints.

IMHO the airlines won't come up with their own policy to switch over luggage checking fees until someone forces them -- right now, it's a win-win for them...both airlines getting the fees.

OP, also AA should NOT refund your checked luggage fee with a AA voucher -- it should not be something that forces you to spend it on AA. It's not a courtesy, it's a fee for a service (carrying your luggage in their hold), that AA didn't provide.