My preferred management upgrade method actually doesn't include any upgrades:
- checking the R7x, R8x upgrade map if direct upgrade to target version is possible
- copy migrate tools of target version onto the appliance
- migrate export on appliance with migrate tools of target version
- build the VM with target version and migrate import into VM
- test, evaluate, document proper functionality
- migrate export from VM
- rebuild the appliance with target version
- migrate import into the appliance, final tests
Related: Upgrade / Download Wizard
In case the appliance was previously migrated from an older version before I'd try get rid of any migration relics by exporting the policy to XML/CSV and importing it directly into a fresh VM installed with the target version.