A new early access update is headed to Baldur's Gate 3 soon, and be warned: it will break save file compatibility, requiring players to start a new save to play with the patch. You will be able to revert to the previous version of the game to continue an existing save, mind, just without any of the newness. Yeah it's a hassle but it's to be expected when playing an unfinished game in early access, really.
The developers explained that the upcoming Patch 3 will bring "the first changes that affect story". So, "Unfortunately, this means Patch 3 will introduce the first save incompatibility, transitioning to the new version."
For folks who don't want to start over every time a patch breaks combability, Larian plan to add the previous version to Steam as an opt-in fallback. That'll be hidden in the beta branch options. You might want to disable automatic updates for the game too. On GOG, I assume you can just choose not to download the update.
Stadia won't have this option ("for the moment", at least), so the six people playing on Google's cloud gaming service will need to start over.
It's not a surprise that this is happening, really. Baldur's Gate 3 is a complex game and Larian still have a whole lot to add and tweak, so it'd be a miracle if they managed to do it while still having a save file work across all patches. However, I will note that Larian's early access FAQ only notes that "saves made during Early Access will not be compatible with the full version of the game when it releases" - it doesn't say anything about losing saves during early access. Still, it really was to be expected. That's early access, baby; don't buy in if you're afraid of things breaking.
While Baldur's Gate 3 is far from finished, it is a good'un if you can't wait. You just might be better off waiting.
"If it were not for this job, and having the self-control of four year old at a birthday party (or games journalist presented with a plate of miniburgers at a press event), I would would want to see this enticing story play out in full," Matthew Castle said in our Baldur's Gate 3 early access review. "Don't feel pressured to gather your party and venture forth, but based on what I've seen, I think that party is in for one hell of a ride when they do."