Is the new vCenter server going to be a physical machine?
If it's virtual you can to a migration & upgrade to the VCSA: Windows vCenter Server 5 5 Conversion to VCSA 6.5 including Update Manager - YouTube
If it's physical (or virtual) then I believe that you need to follow this KB to move the SQL DB and SSL certificates Migrating vCenter Server to a different host machine (5850444) | VMware KB
once this is done, you can upgrade the VC
Hope this helps!