The engine will be able to read the old file format, after you update the array, it will write the .sav in the new format. It’s automatic.
What don’t you understand? 
You need to be able to look at the save file to see what’s going wrong. You can either do that with A LOT of print strings, or you can get a copy of that plug in.
Take a look at the old save file, then run your code and take a look at the new save file. Did it work? If the save file looks ok, then your code is making an assumption that is not correct. It could be either.