Sorry, didn't register first time around.
I've never done a full HA upgrades myself, but this looks to be the safe approach:
1. "Migrate Export" using old version of migration tools on old active cluster member
2. "Migrate Import" on the same version in intermediate VM environment
3. Install R80.10 migration tools in VM
4. Run pre-upgrade verifier
5. Remedy all conflicts discovered by verifier
6. Perform migrate export
7. configure the new cluster for full-HA in isolated environment (you can use offline CPUSE packages to get the version up to date)
8. Perform migrate import
9. Swap the units in production environment