It is EON and I am still testing my "way around" this glitchy save system. So far it works with my way around but I guess we will see true test after 0.01 and then 0.02 updates when players do all possible things that never crossed my mind
From what i have seen in the Visionaere api and in the board regarding these issues your problem should come from you setting new variables and fields the old game didnt have.
The answer to that is make dummy fields / variables you can use later for saving stuff so that old saves are compatible with new ones.
You could also split this game internally into segements so that you can setup default values for a certain progress of the game and the player should have by hand. That could be achieved with one global var and a lot of documentation what to set for what stage.
You could also check the loaded save data for the field it contains and if fields are not set set them with defaults.
That would also take a lot of handwork and documentation due to the size of your project.
But it would allow you to kinda test certain areas of the game faster easier isntead of having to progress through the whole game every time.
I dont know how far you have to go with this game from this point on as last i checked a lot of the statues and stuff were actually completed so i can see that it may be to just see this through now but do it on the new game.
If guess a full license is nessesary to be able to have a look at your poject so i cant help you there.