There is a mod that unlocks them, and it appears as though Thier class is locked. Which makes sense. The only characters I would think should have access to more than one class in the beginning of the game are Astarion and maybe Laezel

Larian's Biggest Oversight, what to do about it, and My personal review of BG3 EA
"74.85% of you stood with the Tieflings, and 25.15% of you sided with Minthara. Good outweighs evil, it seems."