I would think there will be some significant story changes . . . otherwise it wouldn't break saves. Or rather be worth pissing people off by breaking saves just to make minor adjustments
Wont know until its out but it does seem to imply significant enough changes to the story to brick old saves. Notifying that it'll brick the saves is pretty good communication, saves people time on their rage. Would you like a full walkthrough of the new content before its released?
I'm not sure why you think save game incompatibility would have to mean story changes or additions. There's a lot of behind the stage coding that could have been changed in ways that would break saves beyond user-end content being changed or added.
