tbh, we still dont really know much about how larian's origin characters are going to be implemented if player selected or how much control the player will have over making adjustments to character features both at character creation or when leveling up during playthroughs to really say either way. early on larian intimated that multi-classing would be a bg3 feature (possibly even during ea) so i would assume that any character (pc, npc, origin or not) would be able to lvl into any class the player wants contingent on their ability scores meeting the prerequisites - but this may be a long way off in implementation and frankly idk if larian will follow the 5e approach here.
it also raises further questions regarding how much larian is going to allow players to tweak origin character features (race, class, skills, ability scores, etc all have been suggested in earlier threads) - normally id be pro-player choice, but i think too much optionality here would be a significant resource investment that id argue would be better served towards other feedback topics. that said, larian may be more inclined to opening this up if the studio keeps the planned act1 party/map locks and current max 4 party size mechanics in place - but, again hard to tell based off of news from the recent panel discussion. maybe the eventual patch 4 notes will shed some light on this topic
