Not sure if it pertains to this thread, but was unable to change from 12A to 10C on a MD88 yesterday through the app, after a DM got bumped! I was #1 out of 8, with 0 seats remaining! However, since it was very close to departure (they just started boarding the PRE-BOARDS), I assumed that the GA took control and I won't be able to do it anyways. I politely asked, and she gave the seat to me! Probably not the app, I guess?!