I setup an Octopress project following the given instructions (http://octopress.org/docs/setup/) which have you create a Github repository, and create a local repository on
I'm unaware of any way to turn your blank github copy into a github fork after the fact, so
If there isn't a way, can I safely delete my Github instance of Octopress, fork the original on Github, and then add a new remote from my local repository to the newly forked Octopress?
is the way to go. Delete your own github repository, hit the fork button on the octoprocess repository and clone your fork to your local machine.
If you've already made any changes that you with to keep, I'd say that using git format-patch to store them as a series of patch files and git apply-patch to apply those on a clone of your new repository is probably easier than using the old one as a remote (on your local machine).