Just to be clear, nothing with your reservation will change on October 17... what's changing is that all flights that used to be US Airways flights up to that date will become American Airlines flights after that. That's it. Since you've already bought a flight marketed and operated by AA (as it is operated after that date), everything stays the same for you. Based on your OP, it seems you've bought Main Cabin Extra seats as well. This aircraft does have Main Cabin Extra seats, however they are just designated extra legroom seats in the main cabin, not a specific set of rows like we see on other aircraft. For you, if you've paid for Main Cabin Extra (or are otherwise eligible for it) you can log in to view your booking and change to available MCE seats in row 5, 9, 10 or 27 (not all seats in those rows are MCE, but those rows all have MCE seats in them). Pick those and you'll get more legroom than a non-MCE seat (with varying degrees of extra legroom), which means you'll get what you've paid for.