There's no reason to switch. The A321T is the nicest domestic narrowbody plane that AA flies. Even in coach (seatback IFE and power in every row). You will get a free meal. You can't do better than this in a domestic coach product. If this aircraft does not meet your standards you should not be flying coach on any US domestic flight. As others have mentioned the 321T only has about 100 passengers, so even though you will be in coach the plane boards and deplanes VERY quickly, there is plenty of overhead space, and you won't have to wait in a long queue for the lav. It's also a premium route, so unlikely that the flight would be canceled.
As far as selecting seats, you should ask through the travel agent. The seat map blocks a lot of seats for premium customers only, which is why the flight looks full when it's not. However, many travel agents can access these seats (I used to have one such agent).
Enjoy your cruise and don't worry about this flight!