Blair,
If you have taken a backup of the database before you started the database and also the SUM directory then you can easily start the downtime phase again.
You need to restore the database backup and delete or rename the current SUM directory and put the one you have backed up and start the downtime again.
Remember, you will also need to put the old profiles and kernel used by the system before the downtime. You will isolate the CI before starting the downtime as mentioned in the upgrade guide and start the downtime phase.
Cheers
RB