I just started playing with 5.1, having upgraded my Workstation 9 environment over the weekend. One of the first things I noticed was I could no longer convert a pre-5.1 template back to a virtual machine.
I have not looked up the how or why on this one, I just cloned the existing template
Giving it a new name (and in the process picking the alternate datastore I was intending to migrate the original to)
and then (after the clone is complete) delete the original template
And note now if I needed to I can convert the new one.
note also the new one isn’t hardware version 9 or (seemingly) have any other change from the original.