increasing save game variables

Hi.

I have a game where users currently hold a number of saved games. I would like to increase the number of variables in the save game and store extra data in the next build but I have found that none of the save games read correctly after the update, most likely because there are new variables in the SaveGame -extended class.
The old variables have not been renamed or changed types, the internal data structures have not changed either, only added new variables to the class.

How can I avoid losing the old data? I can’t kill off everyone’s saved games every time I want to add data…

Best Regards