You can not change the actual modem mac address since that is what the cable company uses to know the device is tied to your account. If you have a modem/router device you might be able to change the routers mac address. Been a long time since I used a combo unit.
Partially this depends how the cable company has set things up. You should be able to test it by plugging your pc directly into the modem and change the mac on the pc.
In some cases the only way to get a different IP is to leave the modem off for long periods of time, sometime you have to leave it off for a full 24 hrs.