It's definitely possible to keep your upgrade if you change via the app, whereas calling in to make a change presents a real risk of being downgraded back to the originally purchased class and being placed on the upgrade waitlist.

When you're already confirmed up front, whether paid or upgraded, the system will search for the same inventory when you attempt to do a same day flight change through the app. If available, you can confirm it and keep the same class of service. For instance, if you're in 'R' class because you've received a complimentary upgrade to J, you can preserve your upgrade by attempting to switch to a flight that has 'R' class available. However, if the inventory is something like J1C1R0D0I0, as opposed to J1C1R1D1I1, you'd need to be in full 'J' or in 'C' class to be able to do a same day flight change and stay in Biz, and you'll get an option for standby vs. a confirmed seat.

The same applies if you've purchased a discount inventory premium cabin seat. Let's use 'I' class, for example. You can switch to other flights that have 'I' available, but not a flight that only has higher fare classes like 'D' open. In this scenario, I've had the opposite experience -- the app won't allow the same day change, but I've had a phone agent who was willing to make the change when the same lower paid Biz inventory wasn't available.

Bottom line, when you're ticketed in a premium cabin, the capacity control for same day flight changes in the AA app is all about the same fare class inventory being available on the new flight. It's different than the procedure for main cabin same day flight changes, which use an apparent hidden inventory (or previously 'E' class) that is/was necessary for same day flight changes.

As above, this is probably what will happen if you call and speak to a reservations agent.

Yes... key difference is app vs. phone agent.