Have you changed the Cluster version to R80.10 AND Unchecked the box "For gateway clusters, if installation on a cluster member fails, do not install on that cluster"?
If the above two steps are done and changes are published, you should be able to install the policy from the SmartConsole.
It will fail on the cluster member running R77.30 and will succeed on the cluster member running R80.10.
Monitor the progress of the policy installation by clicking "Details" and watching it being applied on individual cluster members.
Then follow the "connectivity upgrade" instructions to complete the upgrade of the cluster.