Very rarely is it desirable or necessary to go back to a prior release.
Should that be necessary, the previous version must be reinstalled and a configuration backup from that version must be restored.
The firewall must be power cycled to complete the upgrade.
Many users have done this upgrade without seeing any caveats other than this, but it is not recommended.
Those with a pf Sense Gold subscription may also use the Auto Config Backup package to make a manual backup noting the reason as "before upgrade".