there must be a way to change a character's subclass, for example, a quest so you can regret the subclass and change it

i think many players play in a subclass that they don't like it, just because they don't want to level up from the zero again

Dumb idea . are you trying to ruin the game or better it ?

If there could be subclass change it has to be under lvl 70.

I highly disagree, players know the risk and KNOW that the subclass they choose is permanent. They have to learn to adapt, not just switch over, that ruins the whole concept.