Before any such operation you need to evacuate the cluster nodes one by one for the operation to be completed. Doing unplumb on NICs may give unexpected results in active cluster, mainly the cluster will panic as it will not find the member available.
You should be able to replace adapters in an IPMP group one-by-one without affecting the cluster operation.
BUT: You must make sure that the status of the new adapter in the IPMP group gets back to normal, before you start replacing the next adapter.
Solaris Cluster only reacts to IPMP group failures, not to failures of individual NICs.
Note, that IPMP is only used for the public network. Cluster interconnects are not configured using IPMP. Nevertheless the same technique can be applied to replace adapters in the cluster interconnect. You need to use the clintr command (IIRC) to replace individual NICs. Again, make sure that all the NICs of the interconnect are healthy before you continue replacing the next adapater.