Unfortunate, you can't revert if you accidentally buy Advance Orb, nor can you downrank spells.
My mistake for buying it in the first place, but that really needs a "whoops, I don't want this 'upgrade'" option
I really like casting frequently, and don't like cooldown / cost increases - so that for me was a big oops.
Appreciate it's necessary though, just bought it sooner than I wanted