As you probably know, you can rename your account or organization at github. Once.
Here are two things you might not know:
1. If the name you long for is already taken but the account seems inactive, github is pretty quick to delete it and offer it to you. You just have to ask.
2. If you're using github pages, make sure to not only rename your page repo, but also push a change index.html once so it gets published at the new address.