upgrading is straightforward. The license is the one of the host and you do not need to reactivate. You do not need to touch the initial ISO setting. Just boot on the new volume inside the guest (as for non-vcmp). Check the resource provisioning after booting.
I never had to. Older (v11.2) F5 doc states (as "might") it's best practice to reactivate the host license prior to upgrade the guest. In newer document version, that statement disappeared. also check http://support.f5.com/kb/en-us/solutions/public/15000/000/sol15029.html?sr=35636906
if i read that first SOL 15930 it says
Before upgrading a guest, ensure that the new version supports the hardware and is supported by the vCMP host version. For information, refer to SOL14088: vCMP host and supported guest version matrix.
Before performing an upgrade, you should re-activate the license on the vCMP host system. The vCMP guests retrieve their license from the host, so you do not need to re-activate the guest licenses.
which says to me do the re-activate on the vCMP host, even when upgrading a vCMP guest only. which makes sense when you would jump ahead of your vCMP host version, then the service check data does come into play.
Be aware, that re-activation the license of the the vCMP host affects the operation of all vCMP guests running on the host.
After applying a new license to the vCMP host you will notice service interuption for all vCMP guest systems. Thats why its recommended to run this in a maintenance window with planned downtime. As mentioned before by Kel you may want to check in advance, if it is really necessary to re-license the host.