We are trying to migrate Intel Flex License server from old server to new server. I have the host id and hostname of the new server. My question is "How to make current license server and new License server work during this transition period"
There isn't a way to change the license manager server without some amount of downtime to update license files on the server and clients. The only way to have both servers using the same license at once is to set up redundancy servers, described here: https://software.intel.com/en-us/articles/change--serverhost-data-node-locked-or-floating-license
You will need to provide a host ID and hostname of a third server to establish a quorum. At least two of these servers must be running the license manager in order to serve licenses. This also requires that you add the second (old) and third servers to your client licenses, such as:
SERVER <server-name1> <hostid1> <port1>
SERVER <server-name2> <hostid2> <port2> //comment: needed for multiple servers only
SERVER <server-name3> <hostid3> <port3> //comment: needed for multiple servers only
If you take the old server offline, then you will need to make sure the third server is running or switch back to using a single server.