I rolled a Paladin but I want to be a Warrior now, wah wah let me pay money to convert so I don't have to level again.
That's why you roll a druid, so you can be a tank, melee dps, ranged dps, or healer.
Honestly, I don't see an AC change being a big deal. Your story doesn't change, your resource doesn't change, your main stat doesn't change, your buff doesn't change, the majority of gear you use doesn't change. You even share a lot of abilities. The only major change is Warriors/Knights having two different armor classes - the rest don't.
I wouldn't want this to be a thing where you can just swap back and forth at will, but it would be nice if you had the option - have it cost a lot of money, put a cooldown in on how often you can change it.