Not quite...
Updating an old save file is not really the way to go. Personally, I would be a
little more forgiving of a missing value when reading in the save file.

something like...
read value (variable storage, default if missing)

I've seen similar algorithms used in code libraries handling registry/user
settings. The definition of the "default value when missing" should be done when
a change to the struct is made.

Let me see what I can do for that.