Based on what we see in other areas of the game, I would guess the hero’s class is not part of the information the game stores about the team. By which I mean, from the game’s perspective, a team is banner + troop/weapon 1 + troop/weapon 2 + troop/weapon 3 + troop/weapon 4. And if you’re using a weapon, it uses whatever class the hero is at the time of the battle.
So if you want to use a different class every day, you want to change it yourself at the start of the day that you want to use it on. It would also mean you could change the hero class in the middle of the day and have that change reflected in your defence team.
Again, just a guess. Looking forward to official confirmation