Sure it's possible, however the only way to do it afterwards is to delete the repository and fork it manually again. Deleting the repository is only fine if there are no issue/wiki/data associated with it.

The main problem is not omitting the "fork the repo on github" part voluntarily. Sometimes you're just not aware that you're pushing a repository which already has some ancestor on github, while you originally cloned from the main author's website instead. This has happened to me countless times.

The graph network is totally useless in these cases.