@mspence It is always a best practice to clone an instance to a sandbox instance before upgrading it to a new release. You can always use the clone of sandbox for restoring afterwards if anything goes wrong after upgrade.

If you didn't clone the dev instance in this case the only option left is to reach out to ServiceNow support by raising a case from the Hi portal and request them to restore the instance to pre-upgrade version.

Since ServiceNow support maintains a backup of the instances for some time they may be able to help you here.