If your server IP alone is changed and not the primary ethernet MAC ID/HOST ID it's an easy job.
First check out if your license server is UP in the server maching by checking the ptcstatus in command prompt. If the resultant message is SERVER UP.
Then try to run reconfigure.bat (if it's Creo) else ptcinstall.bat (for Wildfire versions) at the installation/bin folder in the clients machine and you need to update the new IP adress in the server screen along with the port which usually would be 7788 and finish off the re-installation process.
At times if your server IP might not be recognised in the client machines and try switching off your Firewall in the client, and add up host entry of your server machine in the client and it should help.