Migrating Sql Cluster Installation To New Drive
Your steps to do this are pretty much right, I've added some steps and changed the wording on a few items.

Some SAN vendors provide migration software so you only have to turn off the cluster nodes for a few minutes and they will migrate the data behind the scenes.  You take a breif outage and a performance hit until the migration is complete, but you don't have to manually copy the data over or have to change the drive letters.  It's worth looking into for having 13 instances.


Create LUNs on new SAN.
Present new LUNs to the cluster.
Copy everything from old clustered disk resource to new clustered
disk resource.
add new disk to the cluster group
change old disk letter to an unused disk letter or remove disk letter
entirely
change disk letter for new drive to be the same as old disk letter
change dependencies in the cluster resources to the new disk
remove old disk from cluster group
bring SQL resource online