You might run into 2 snags: some airlines insist on reissue of ticket (and collecting the change fee) even though it is not needed (AA does this, while BA and QF usually accept the flight coupon). The second snag is that codeshare inventory is separate and not transferable (also seat assignments usually are not transferable). Typically AA buys very few seats on codeshares and doesn't sell many at a discount; so you are quite likely to find plenty of BA inventory and zero AA inventory. Your existing seat cannot just be transferred, and often the codeshare partner is allocated all the crummy seats on the plane.