I wouldn't assume that; with a change in cabin, your ticket needs to be reissued or revalidated ; AA doesn't seem to tend to do revalidations so will probably need to be reissued ; if booked into 'U' class, then AA should be easily able to cancel existing award , collect taxes, issue new award and issue a ticket

I am lost on your second question - if you cancel your booking instead, then you will simply get a refund of miles and taxes