I am running smf 2 rc1, is there a way to migrate back to smf 1.8?
you currently can not convert 2.0.x to 1.1.x or 1.0.x.
No, it's effectively a one way process.
If you have a backup, which you should have, of your forums files/folders and database before the upgrade. You will be able to restore from that to your previous state.