you can get and play your old saves - in steam select the game right click and go to properties. Then betas, under beta participation select previous_version.
You'll be playing on yesterday's version of the game, but you can continue your saves. When you finish your game would recommend removing the previous version selection and that'll put you back in the newest version for your next game.
This really... really... REALLY should be a pop-up on 1st launch of a new update if that update isn't compatible with old saves...