Who cares? I mean we have to level up a new subclass anyways. If we reset back to 20 it just means my grinding will be for multiple things.
Hell if you prep/store up 5000 exp bounties and daily event packages before TTK drops, you'll probably go up a couple levels right off the bat.
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First you're gonna have to unlock the new subclass by completing a quest line. You won't be levelling up the new subclasses straight away