I remember earlier this year when I looked at a Universal Annual Report that the Shrek deal is on a year-by-year basis. So, it isn't hard to get out of.
They don't need Shrek anymore. I think a big part of the Shrek deal was needing a marketable character, especially for USF. Now with Potter, Despicable Me, and Transformers, that is no longer necessary. As to what can replace it, I have no idea, but like Skip said, they're building an e-ticket where SS44 was.