Major Media Center versions are separate installs, so when you install Media Center 27 it'll be installed alongside Media Center 20, which means you can run both version on the same computer (though I would recommend not running both at the same time, especially if you're using Media Network).
Just install Media Center 27 and run it, import a library backup from Media Center 20 if Media Center 27 doesn't already do this when you first run it, and give it a go. Then you can decide what to do, you can leave Media Center 20 and not use it, just in case you want it or if Media Center 27 does everything you need, you can uninstall Media Center 20.
