You don't have to replay any old missions to unlock to the vital new story missions. Just do a couple side ops (including all the **important** side ops), and listen to the important tapes, and the new missions will unlock.
RE that last paragraph: as someone really excited about the weird, experimental stuff that'll come out of all this, I'm more excited about trying dozens of small projects a month than any high profile games, at least in the first six months or so. I want to be there while devs figure everything out.
Draekwon's comments