Recently, a github project that I contribute to, joind.in, moved from an ordinary github user account over to an organization. Getting contributors moved over is pretty straight forward, I have a fork of the main repo on github at http://github.com/lornajane/joind.in and that updated to show itself as being a fork of the organisation's repo rather than the original user repo that it had been set up under.
In fact, all I had to do was update my upstream remote on my local repo - I set this up following the excellent github forking instructions when I first forked the repo. All I did then was to check my remotes:
This showed my remotes with the "upstream" pointing to the old repo. So I copied the URL of the organization repo, removed the old version and added a new upstream:
git remote rm upstream git remote add upstream git://github.com/joindin/joind.in.git
Everything now behaves as before while handling the new central repo for the project - hopefully this helps others with projects moving from user accounts to organizations (or organisations, as I keep typing, British spellings as always!)