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First, make a plan of how many new disks each node in the entire, upgraded cluster will have. Typically, one or two new disks will be on each node. Read the next steps and think through exactly where each disk will eventually go in the cluster.
Next, add the new nodes node(s) to the cluster but only populated with the empty disks each node will have.
Iterate over all the remaining cluster nodes by taking it down using normal shutdown operations. This allows the cluster to clear any client requests from the node being rebooted. When the node is down, pull however many disks that need to be replaced and put in empty disks. Bring the node back up. Meanwhile, hot plug add the 1-2 disks to the same new node. These steps can be done in parallel. At the end of each step, you will have added a handful of empty disks while shifting the location of existing disks.
Repeat for all the remaining nodes.
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