I just upgraded our vCenter server from version 6.5 to version 7 update 1. The problem is after upgrading I can not assign my old vCenter license to new server. Also I have the same issue with vSAN licenses. I attached a picture of my License page to this message. As you can see in the attached file the first row is the Evaluation license for vCenter server and below that you can see vSAN licenses that are unassigned.
thanks so much for your help and support.