This article from 2007 does say that the Skins are bound to the land lease until 2027, but even in it, they said that discussions on a new stadium could start around 10 years from the article (we are at 9 years now).
The land lease aside, I tend to think the question "What to do with FedEx" is really a moot point in terms of what the Redskins are going to do. Whenever they move out they will write off Fedex Field to the relics of history and not look back. My guess is they will make a big show of watching it implode, and say goodbye to that part of Redskins history.
