I have some vSPhere 4 standard licensed hosts in vCenter that had the key entered during install. I now want to change the licese key for these servers to use new Enteprrise Plus licenses that are controlled by vCenter, but I cant seem to do this. I have tried removing the keys but it won't let me, if I try to assign new unassigned keys they are greyed out within vCenter (they are fine to apply to new hosts).
This is the official procedure and should work:
In vCenter Home Screen -> Licensing.
Select one if your hosts, right click and select 'Change License Key'