I had a customer upgrade. It did in fact show the IOM3 as unsupported. I think the other side effect is that you are no longer able to get correct environmental information from the shelf.
It just got too scary for me!
I suspect what will work: On your A220, use the serial console and configure the Node IP. Then break away to the CLI (skip cluster setup). Find out one of the "Cluster LIF" IPs on each of the new nodes. (net int show -vserver Cluster)
Then run a command like this from your current cluster
cluster add-node -cluster-ips 169.254.204.213, 169.254.234.125-allow-mixed-version-join true
where 169.254.204.213, 169.254.234.125 represent one of the CLUSTER-IPs from each of the A220s.
Migrate to the new controllers. Then "cluster remove-node" of the older units. The cluster will likely jump to whatever release is on the A220s