Have the same problem!
I asked for an upgrade to 4.5.1, but have not been able to license a single copy using our existing license number. Now an attempt to re-license existing copies of version 4.0 that have been unlicensed, results in a message saying all licenses are used up.
Do all 4.0 licenses have to be uninstalled before 4.5 can be used?
What am I doing wrong in trying to activate our 4.5.1 versions.