One thing that can help, is to remove or change the client's record of the old IP address.

Normally, the client saves the DHCP address (or "lease") granted, and requests the exact same address on subsequent registrations.

If you can find the file where this is stored and change it, then the client should request whatever you put there in place of the original value.

Whether or not the server honours the request is another matter, but that would depend upon the individual server.