Saved Game Variables

Hi Folks,
I have a project that exposes different skills/ assets according to how much total game time players have accrued,
the variable that drives the exposure “TotalGameTime” is held in a game save object and all is working as it should.

However I have a problem :-
I will be updating / expanding the game over its lifetime, and the default value for TotalGameTime would be zero.

So if I package up an update and release it … it would reset everyones TotalgameTime… they would not be happy.

How can I get around this?
When you install an update can it add to an existing package or does it need to overwrite everything? ie its a complete fresh install

Thanks Paul G
(should this be in Modding forum ?)