I want to expand the structures with some "at the moment unused" variables which I save with dummy values. In addition I will create some "dummy" structures which will also be saved (just the amount, no data).
IMHO, the "micro$oft approach" is better. The first field of a structure is the structure's size. If you add some new fields, the structure that you read from the file will be shorter than the current version of the same structure, and you just fill the missing "tail" with zeroes.

Why is it better?.. You won't limit yourself with pre-set structure sizes, always able to expand them as you wish.