Last week I was building a cluster and it initially had trial licenses on the hosts. I was using ESXi 5.5.0 1331820 and the vCenter Storage Appliance 5.5.
I had just built the cluster and like to take advantage of the features of the trial license during the build process. Storage vMotion and other features can be used during this period.
Makes it easier to move things around while the cluster is being built and tested.
Now that the cluster is going into production, I was applying the ESXi licenses, which were Essentials Plus to the hosts.
It keep refusing to downgrade when applying the license key through vCenter.
The problem came up that it was stating that DRS was in use on a host (which it was not) I also checked, storage I/O control was not enables on that hosts storage.
I moved all the VM's off the host and put it in maintenance mode, and rebooted it. Again no luck.
Eventually I removed the host from vCenter. Then I attached via the client directly to the host. I then entered the key into the stand alone host, and it took.
Then I reintroduced the host to the cluster.
Hope this help if someone else runs into this one.