This is not quite the same issue.
In this case you have to write the game code to cope with both the old and new versions of the save file.
What you have to avoid, is changing the name or types of variables. I don’t think there’s any way back from that.
I would also make sure you have copies of the old save file to work with, make sure your new code works with them as well as the new format.
For instance, you’re talking about having an array of ownership, but are the new items just added to the end? You haven’t change the order of the items you had in the old version, I assume.
Also, I’d recommend this
You can open the save games and take a look at what’s actually in them, rather than trying to debug things in another way… 