Thank you all for your feedback.
In this case I will provide a downtime but it is very minimal downtime and it gives me the opportunity to roll back immediately on old cluster member.
Let me explain and share with the community, and also let me know guys what do you think about it:
1. Disconnect R77.20 standby cluster member
2. Connect R80.30 new member with ONLY interface that leads to Mgmt Server
3. Change cluster version, fix cluster member topology, install policy (remove flag "if fails")
4. Disconnect R77.20 (DOWNTIME)
5. Quickly Connect all the remaing interface on the R80.30 (check arp table and clear them if needed on network equipment/routers connected to R80.30 gateway)
6. Verify that everything is working fine -- END PROCEDURE
**In case there are issues you can switch back to the R77.20 cluster member
I know this will interrupt all the connections and the customer has to agree on this one.
At least you will have the ability to quickly switch from one node with old SW version to the new one and viceversa.
This is basically the same procedure I have applied with cphacu in other situation.
In this case we cannot use it as we already discussed it, and so I think this is the only way to do it.
If someone has other idea/solution it will be helpful.