This is one of our biggest issues with AGS and is the main reason why we don't update our games as often as we'd like. 
The main rule of thumb is to change script in ASC files *only*. Changing script is usually safe. Adding or changing graphics is not. As the others suggest, I'd create about a dozen or so dummy global variables of every type before you launch. That way if you need a global variable later, you can use it.
There are other tricks, like reading graphics from external files instead of importing them into the editor. Try and avoid that if you can!
If you do make an update, keep a few savegames from the previous version handy so you can test that they still work.
Good luck!
-Dave

The main rule of thumb is to change script in ASC files *only*. Changing script is usually safe. Adding or changing graphics is not. As the others suggest, I'd create about a dozen or so dummy global variables of every type before you launch. That way if you need a global variable later, you can use it.
There are other tricks, like reading graphics from external files instead of importing them into the editor. Try and avoid that if you can!
If you do make an update, keep a few savegames from the previous version handy so you can test that they still work.
Good luck!
-Dave