That depends on what you want to do with the old server. Is it being decomissioned? Then simply shut down/uninstall the services and install the license on the new server. Or first Clear the license, but that is not really necessary.
Is the old server becoming the new production server? Then install the new license on the old test server, and install the test license on the new test server.
Never run two QlikView installations using the same license in parallel.
But if the same scenario happens in Production which I need to includes the configuration from Old production server to New production server, then I need to modify those config xml or ini files, right ?
do the config files include those setting such as tasks schedule, paths setup ... that i set in the QMC ?
Yes. Follow the instructions from the Migration document. It contains an entire chapter about server-to-server migration.
Yes. Everything is included in what is commonly called the QVPR. It has all configuration settings and all task definitions and more. You can find the QVPR on your QlikView server in folder C:\PRogramData\QlikTech\ManagementService\QVPR