Revamps are bound to occur from time to time and big ones, like the one we just had, triggered quite an uproar and devastated the impact of pure damage-dealing spellcasters. Players that have invested heavily into their elementalists or even multiple copies of it via limit breaking are naturally upset that the efforts they go to naught. Even heavy spenders would be deterred if the only way they can stay on top is to have a lv40 copy of every valiant classes, and more if new different valiants (anti-valiants or whatever) are released.
To mitigate such impacts, I suggest allowing players to switch hero classes but with limitations, such as limiting it to once per class per month or on a rotational basis (2 specified classes for a particular month). These are just suggestions and you're free to suggest other forms of limitations, but allowing class-switch is very necessary to allow players to fully commit resources into their units without the fear of them being completely phased out by 1 patch.