I might be wrong here, but if I remember correctly there is an interview (or was it the panel?) where Swen said we would be able to change a character's class when we pick/recruit them, like in DOS2. The idea being about flexibility when building a party from a relatively small selection of companions.
I hope I'm right. Too often, in RPGs, we end up picking the main character's class for the way it complements their companions, while the opposite feels more enjoyable to me. Sure, canon classes make more sense from a roleplaying perspective, but then again, nobody would force us not to use them.
Assuming that is correct, how would that work when a character’s class is so integral to their personal quest? I guess we will FnD our either way soon.
It really depends on the technical necessities of the tasks related to said persornal quest, and wether another character can do something in their stead. Let's say you need to cast a specific spell to complete a quest, maybe the game will juste give you a scroll for it, or maybe any spellcaster can do it. It's a roleplaying game, there are options.
And it's fine with me if it makes some personal quests harder in some way. It would be a reasonable sacrifice to make, I think, so your party can include both the personnalities and the classes you want to have around. Especially if it's, let's say, your fourth or fifth playthrough.
But yeah, we'll be sure soon enough and if classes are set in stone it's not exactly an issue or a problem.