You can add new variables without any trouble as long as you make sure to declare them e.g. at the start of a new chapter or wherever they're going to come into effect. Of course if it's a new variable that you want to add into already existing content that's a different matter.
Yeah, and even the latter can be handled with a bit of immersion breaking and care. For example,
1. Declare the new variable with default (as it should be done anyway) and set it to some value it won't have latter on
2. At the edited content, change the variable depending on the decision
3. In the new content when you want to refer to the previous decision, check first if variable still has the default value, and if so, ask the player what decision they made
Of course this only works if the decision has to be made, if it's for example a decision in an optional event in a sandbox (or the new content can be seen before seeing the old one), that's a bit trickier since you'd have some way of knowing whether the player saw that event (possibly by checking other variables) or ask the player if they saw the event.
Anyway, Renpy has come a long way, and a significant portion of modern renpy games release episodically, so while one should take care, save compatibility isn't something to be
too worried about