Move Plesk license key

Download the plesk key from the old webserver.
The location of the plesk key is /usr/local/psa/tmp/psa.key

Logon to the Plesk interface of the new webserver and upload the file to the server.
(Server > License management -> upload key)

You can use /usr/local/psa/admin/bin/keymng but we prefer the procedure above.