Obvious troll is obvious, but I kind of want this.

I'd even be okay with it being an MT item, if limited (could only do it once a year or suffer a mental break).

There is precedent. You used to be able to join a new guild if you met certain requirements, such as < circle 2 and some other requirements I don't remember. They would strip you of all of your experience, but maybe there's another way to go about it. What if the skill just went dormant. You kept your ranks in your guild skill, but you couldn't use those ranks. You're stripped of all guild spells and abilities. All feats, careers, techniques, quests, everything. It's like you never did it. Your circle is set to circle 1, and your stats are reset to your racial minimum.

Or maybe your guild skills is wiped and sent to bonus ranks, but that seems like a playability problem.

From a lore perspective, attunement (other than arcane) is innate. So go along those lines.

Clerics <=> Paladins
Rangers <=> Empaths
Bards <=> Warrior Mages
Traders <=> Moon Mages
Thieves <=> Barbarians
Necromancers <=> Necromancers