The launch of Baldur's Gate 3 on Steam Early Access was legitimately exciting, but at this point we've pretty well settled into the long slog of actual game development. That, unfortunately, means it's at the stage where necessary major updates—including patch 3, which Larian said today "is just around the corner"—are going to start having compatibility issues with existing saves.
Patch 3 will be the first to make changes to the Baldur's Gate 3 story, and that means existing saves will not work with it. But, for the benefit of those who'd like to continue their current sessions, Larian is introducing a secondary branch of the game that will maintain Baldur's Gate 3 as it is.
To access it, you'll need to right-click Baldur's Gate 3 in your Steam library, then select "Properties," and then the "Betas" tab. Choose "patch2" from the list, then exit out of the menu. That'll leave you on the current patch, so you'll be able to continue on with your current playthrough. Unfortunately, the Stadia version won't support branching "for the moment".
If you need some time to make up your mind, Larian recommends disabling automatic updates in BG3 by going into the properties menu, selecting the "Updates" tab, and choosing "Only update the game when I launch it" from the dropdown menu. If, on the other hand, you already know that you want to lock it down so you can continue your Faerûn follies, you can opt into the secondary branch now, even though patch 3 itself doesn't actually have a date yet.
"We know that having to switch branches may be annoying, but this is part of early access," Larian said. "All the feedback we've been gathering from your adventures and posts will go right into these patches, so next time you play it’ll feel like an experience you've helped to create."