After reading up on other people's posts and discussions I think it's time to bring my own theory to the forums in it's own thread.
What I think happens when Garrosh is "going back in time" is in fact him going to an alternate reality(or universe), a reality that began approximately around 30 years after our own.
Let's say that the Opening of the dark portal is year 0 and that each game/expansion moves the year by 1(just to keep it simple)
Our reality has warcraft 1, 2, beyond the dark portal, 3, reign of chaos, wow, expac 1,2,3,4 and now 5. that's 11 theoretical years.
their reality only has warcraft 1 and 2 and beyond the dark portal I think... since we do see Khadgar there, meaning he did go through the portal once before. so that's only 3 theoretical years.
Now the Red colored Dark portal will takes us to this reality and merge it with our own, allowing them to spill into our world and us to spill into theirs. And being 2 separate realities it wouldn't change anything in our past, so no retcons required for us. Their future will however probably change, and they will not have the same events that we had from now on.