What does these change?
At a guess, probably the things listed when you click on the link that says "Full list of changes" on the page the above link takes you to.
Can they be applied to allready saved games? Much
better question. They should generally work on saved games, in fact I first heard about them when they were suggested to me as a fix for something that wasn't working right in an MM6 game in progress; I installed them and it worked fine, no issues with the saved game. I can't promise this will be the case for any given other user, but I know of no reason why it wouldn't be.