Does the device provide a client-id?
Maybe that one remains the same.
(IP addresses can be assigned on both client-id and MAC, client-id should prevail, but it should be possible to use only client-id)
http://forum.mikrotik.com/t/dhcp-clientid/53010/1
http://forum.mikrotik.com/t/dhcp-mac-vs-client-id/147910/1