How to downgrade IBM V7000 (Storwize) firmware

Sometimes, especially when you do a problem management, you have a need to downgrade firmwares on some system components. I have such need for IBM V7000 storage array. Downgrade process is not documented in IBM official documentation so here is the downgrade process step by step:

1. Double check you have IP addresses on management interfaces of both canisters (controllers)
2. Login to management interface of one particular canister over https. https://[ip_mgmt_canister]. You have to use superuser credentials. Default IBM Storwize superuser password is passw0rd
3. Switch node to serrvice state. You should wait 15-20 minutes
4. Login to second node management interface. 
5. Switch second node to service state. You should wait another 15-20 minutes
6. Double check both nodes are in service state
7. Login to one node and choose action "Reinstall software". Browse and upload firmware image via web browser. Software reinstallation takes a while. You have to wait approximattely one or two hours. In the mean time you can ping canyster management IP addresses to check when nodes comming back. 
8. Repeat software reinstallation for second node.
9. Please be aware that storage configuration is lost after software reinstallation. Therefore you have to use default password for superuser. Recall it is passw0rd
10. When both nodes are up and running login to one canister node management interface and exit both nodes from service state. It can takes another 15-20 minutes.
11. When nodes are active you have to regenerate Cluster ID. You have to go to "Configure Enclosure" and enable checkbox "Reset System ID".
12. After all these actions you have Storwize ready to form a new Cluster. So create cluster and assign cluster virtual IP address you will use for standard storage management.