Is your 4.5 server x86?
How do you assign your encrypted users? manually or add local domain users?
Do you have any clients on the new server at all just now- if not read the discussion and procedures posted by RGC here: https://community.mcafee.com/message/302134.
(may involve rebuilding your 5.1 server)
I have a similar scenario- try and use that procedure, or this https://kc.mcafee.com/corporate/index?page=content&id=KB71078 first. As tranfer of assigned users is not supported. (Now i have two servers with encrypted machines on them and its turning into a nightmare!)