I have 2 questions.
1. How big of a change is the ending/how much extra content is there getting it?
2. Are old saves compatible with the new version (and will picking one of the later one work or would that skip something I need to interact with)
ending has a big change. i added more scenes so you can see what happened to the leaders of the other sectors. an alternative ending was added also which bring me to the second question:
old saves are compatible and will work without issues until after the reactor sequence. any save made before that wont have any problems. however the choice you have to make to get the alternative ending is at the end of day 2 when you get to use the compound. so if you start after that, you'll miss it