Yep, it will be... a pain, that is. It might also be an opportunity to grow and garner some more publicity with a platform that is as active and popular as GitHub. Of course that would require the conversion from the Mercurial repository to a Git repository -- but according to the linked article there should be tools to do exactly that. One might also want to consider starting anew and keep the Mercurial commit history somewhere saved separately and simply import all current files into a fresh Git repository.
The neat thing about GitHub is also that you can just put up
releases for your repository, so the release *.zip file we currently carry around in the repository and its history could finally be removed, and the big, inefficient binary diffs it has caused over time would disappear too (assuming you plan to create an entirely new Git repository).
Well there's still a bit of time yet, and I don't want to force my ideas onto you again, but I think this would be the ideal time for some neat changes (excluding having to learn about Git).