Hi
Is the screenshot is a new vCenter 5.5?
If yes, its actually stated that the vCenter Server 5 Standard license is assigned (Assigned column=Yes), and the evaluation is not assigned to any vCenter (Assigned column=No).
So you do not need to unassign & reassign since it is already assigned, you can safely remove the Evaluation License.
You do not need to worry about the old vCenter as you are planning to destroy the old vCenter - no need to unassign from old vCenter too.