- Edit your classic Site
- Goto Manage Site > Convert to New Sites
- For an uncompleted migration, you should see option 3 - Review and Publish - highlighted
- Under that there is an 'advanced' section.
- Click on that and you will see a 'disconnect' button.
- Click that button to disconnect the draft New Site from your classic Site and you should now be able to restart the whole process.
Thanks to forum contributor Perjury for this information.