It's hard to imagine how they can be different classes than they are. Gale, Shadowheart and Wyll have their classes pretty ingrained into their storyline, and it's hard to imagine how they could be anything else (excluding subclasses). I guess Lae'zel and Astarion technically could be different classes and get away with it, but chances are if it's not possible for some of the origin characters then it won't be possible for all of them.
