.... when attempting to load an existing save after updating....
... the update says that it's not save compatible. 
I assumed that meant you couldn't add this mod to existing saves, but this save was started with the mod, I just updated it mid-playthrough.
It means that it is not compatible with saves, as in you can't update it in an ongoing game (you can't change v1.65 to v1.66). So you can't update it mid-playthrough.
You'd have to start an entirely new game with it already at v1.66, if Tartiflette created a hypothetical v1.67 and it
was save compatible then you could update the existing v1.66 save (as long as you did the proper procedure of deleting the old folder and replacing it with the new one).
Adding an entirely new mod to an existing game can be done (at least if the mod author hasn't stated that it can't), but may have missing components such as custom star systems (those are generated on the creation of a new game) which may cause problems with the mod. I think that covers most things...
