Well as far as I know no one has switched subclasses ever, Yor did not switch sub class he merely stopped using the light so he wasn't any class. The closest thing to someone changing in lore is when felwinter used shoulder charge to hit a warlord but he didn't switch he just said he learned it from lady jolder. So I'm guessing lore wise guardians can learn things from different classes but it probably takes a lot of training.