Not sure I understand the sequence here.
1. OP had a carry-on which fit the sizer.
2. OP had an encounter with a GA which was less than pleasant.
3. OP's carry on now did not fit the sizer.
If that's the case, somebody is due a Nobel in physics and I would led the rest of the issue slide.
Perhaps what really happened is that OP had a non-compliant carry-on and although the GA could have handled things better, the bag was going to be gate-checked so the old tag had to go.
Bear in mind that AA has cracked down on non-compliant carryons of late, especially in the first few boarding groups.