Upgrade Question for HA Pair

A HA Group does need the same modules provisioned but can be different platform types. F5's Device Service Clustering also supports n+1 HA groups so you can realistically add the two new devices to the same sync group and then phase out the legacy ones. That being said, things to bear in mind is they will both need a base setup, as Iain mentioned, plus the TMOS versions would need to be the same and, finally, they will have different host names and mgmt IP addresses - so it depends on if this is a requirement.