I highly suggest contacting your VAR to help you out.
In order to affect this conversion the VAR would have had to be involved. You will have to install the network license server in order to activate the floating license. Andy is right, you need to contact your VAR. It should not take long to achieve this.
If I am thinking correctly, you should have received a new serial number for the network license - these will usually start with 0010.
(most standalone licenses will start with 9000)
Install & Activate the Network License Server, and you should be good to go.
Your VAR will be a HUGE help with all of this - won't take long.
All my SolidWorks Networks licenses start with 9010, my EPDM licenses stare with 0010...
& yes a standalone converted to a networks license would require a "new" serial number from your VAR.