Thanks! Losing armor and weapon proficiencies would be unpleasant but bearable, losing skills very unpleasant, but it's nice to know that this would work in principle. And I know I'll lose access to level 6 spells, which is why I don't know if I'll regret it later. Bit since we can respec, that's not too much of a problem.
Here's where "we don't know how it's going to work" comes into play. No class in D&D gets two bonus actions a round, so there's nothing to compare a BG3 thief to in D&D. In D&D, a character can cast two spells in a round only if one of them is a leveled spell that takes a bonus action and the other is a cantrip that takes a full action. That's also how casting multiple spells works in EA.
In D&D, when multi-classing into rogue, you gain light armor prof (and a skill prof), so if BG3 works like PnP, you won't lose armor prof if you go from sorc to rogue.
