The reasoning for the incompatibility with game saves is that this patch will affect the game's story. This, unfortunately, means that you will have to re-roll a new save if you want to see what is "new" with the upcoming patch.
Fret not; you will still be able to play your old saves thanks to Larian Studios adding in the ability to select previous patch branches. This will allow you to continue with your current save without any of the new additions. You will have the option to choose when you want to re-roll on the latest patch for the game.
Currently, branch selection will only be available on Steam, and those who have purchased the game on Google Stadia will not be able to switch branches, and thus, you will most likely be forced to start a new character.
Larian Studios does mention that saves will never be deleted but will only be accessible with the selected branch that will work with the save. Hopefully, this means that once branching is available on Google Stadia, you will be able to revisit old save files.