The license manager uses the MAC address of the NIC in the server - so if your new server has a new network card, even if everything else is the same, neither FlexLM or the old Elan LM will work - you don't actually say which you are using.
You need to call AutoDesk and get a new authorization code or .lic file.
Depending on the clients, you may also need to update the licpath.lic file mnually. I found that WinNT 4.0 machines will gernerally rewrite their own licpath files, while Win2k machines will not. The good news is that you can do it once, and then copy the file to all who need it.