From the Wiki: http://wiki.mikrotik.com/wiki/All_about_licenses
What is a Replacement Key
It is a special key which is issued by the Support Team if you accidently lose the license, and the Mikrotik Support decides that it is not directly your fault. It costs 10$ and has the same features as the key that you lose. Note that before issuing such key, the Mikrotik Support can ask you to prove that the old drive is failed, in some cases this means sending us the dead drive.