Are you sure? I think Larian just said you can fully respec companions. Link below, and the pertinent part pasted.
https://store.steampowered.com/news/app/1086940/view/3655285307835439472"In Baldur’s Gate 3, you have the freedom to respec not only your chosen protagonist’s class but also that of your playable companions and pre-authored Origin characters, whether you’re looking to make a few adjustments to their existing build, you’ve got a burning desire to experiment with entirely new playstyles and class combinations, or you just want to create an all-badger supergroup. Keep in mind that certain Origin stories are closely intertwined with their characters' respective classes. If you’re opting for a non-cleric Shadowheart or a Wyll without his warlock pact, this will undoubtedly influence your roleplay options in dialogues and the overall narrative for these characters. But their stories will instead be yours to create."
In a recent interview Swen mentioned that you can change everything but two characters' sub-classes because they are tied to their identities. That's Shadowheart and Wyll. You can make them things other than a cleric and warlock, but if SH is a cleric, she is a shar worshiper, and if Wyll is a warlock, his pact is with a fiend.
I would love to tweak SH to light at the begining, too. But it seems like that probably isn't an option.
