I'm not going to go into a long winded explanation of why class swapping needs to be locked in ATs after the match starts.
But it does need to be locked. Swap all you want before the match begins. After the match begins, everyone's character should be locked.
During final rounds of ATs, try-hard teams will swap, and then your team needs to counter swap, and then they swap to something to counter you, and then you have to swap. It's really just beginning to turn into a painful chore in the play dynamic. I think I speak for most people when I say that, we want to spend our time in the final round actually in combat doing things in the match, not in our character select screens.
After an AT match begins, please just lock characters, exactly the way that it works in ranked.
~ I left GW2 on 11/22/2020 due to widespread cheating and the lack of action to moderate it