i can't say anything about running different versions in a cluster, as far as i know it is supported but i have no experience with it. Regarding updating your existing cluster: upgrade is quite easy and not complex, you just have to take care of a few things.
First, you need a Failover Manager. The FOM provides Quorum for the cluster when one of your two nodes goes down. To avoid split-brain-situations, a cluster with an even number of nodes will stop all activity when it is not able to reach more than 50 % of its nodes. With a 2 node cluster, rebooting a single node would stop the cluster. The failover manager acts as a third node (or manager, as they are called in Lefthand StoreVirtual terms) so that your cluster remains online when one node is offline.
Another thing to remember is that all volumes that are created with Network Raid 0 will be unavailable as soon as one of the two nodes goes offline. So if you have Raid 0 volumes in your cluster, make sure the systems using those volumes are stopped or at least can cope with an unavailable volume. If all your volumes are Network-Raid 1, your volumes will remain available during the upgrade.
To start the upgrade, first upgrade your CMC to the latest version (currently Version 10). After that click on "Upgrades avaliable", then on the upgrade page click on the "continue" button next to your cluster. You can then select to either apply patches to the current software version or upgrade your nodes to a new software release. Either way, the CMC will start updating the first node (usually the one that doesn't have the Cluster VIP address - if a FOM is present, the FOM will be upgraded first), wait until it has finished, rebooted and resynced. After making sure the upgrade on the first node worked, the second node will be updated.
We have upgraded and updated our cluster with 4 P4500 nodes several times during business hours and haven't experienced any disconnects yet.
