My understanding of Essentials is that it can Manage 3 Servers and cannot manage more.
It can however decouple servers and couple with new servers, at least based upon my experience up to now.
I will be decommissioning one of my 3 servers so my intention is to migrate all VM's to the other 2, then Drop the Server from the VCenter set and then Join the upgraded/or replaced server again to the same set in its place.
It has been a while since I was working on this, but so far things have been working well.
The "locked" aspect of Essentials seems to not serve VMWare well in that I would be happy to buy a license to a new server or two but I don't want to throw away the license I have now..
All good. Thank you for all the comments.