I would do the following:
Add 2 new nodes (Cluster will have 10 Nodes)
Validate that new nodes are correctly connected to relvant network
Create intercluster lifs (if Snapmirror)
Move all volumes to the new HA Pair 1:1 (that way you will ensure the same level of performance)
Move / Rebalance all lifs to the new HA-Pair 1:1 (that way you will ensure the same level of performance)
Ensure that no volume reside in the old HA-Pair
Clean-up old nodes
remove intercluster lifs
delete aggregates
disable HA
Move Epsilon out of this HA Pair
Evict node by node