AP licenses are model-agnostic and also never tied to a specific AP or serial number. So if you have 10 AP licenses, it doesn't matter what mix of actual access points you have, as long as there's 10 or less. If you are removing some MR16s from a network and adding in some MR42s in their place, there's nothing to do, it's automatic. If you end up with too many APs after putting the MR42s in place, then it would be because you didn't remove the old MR16 APs. Once you do, the MR16s will be sitting in your Organization > Configure > Inventory, but not assigned to any networks and therefore will not decrease your license count. Hope that helps!
