Force new WAN IP from ISP with new WAN MAC address on router

I recently wanted my ISP (Spectrum Internet, formerly Time Warner Cable) to issue me a new IP address. I called the provider and they informed me they couldn't do it manually. The only way for me to get a new IP address is to wait until the routine planned scheduled date (in a month) for a new IP to be issued, or to get a new cable modem (ISP would assign a new IP to the new MAC address). I opted for the latter because they offered me a free modem as part of my plan, and because I wanted this done ASAP. However, the ISP still issued me the same WAN IP to my router when I plugged in the new modem. I next tried plugging my laptop directly into the modem using an ethernet cable. The ISP did issue me a new IP address. This shows that the ISP issues an IP address to the device plugged into the modem, not to the modem.

Thankfully, my router had Tomato firmware running, and I was able to increase my WAN MAC address by one bit by going to my router's config page (http://router_ip/advanced-mac.asp), changing the MAC address, and saving the settings. My ISP saw the new MAC address and issued me a new IP address. In a month, I will revert back to my old MAC address.

NOTE: I originally tried to modify my WAN MAC address using the nvram command:

nvram get wan_hwaddr # WAN MAC address; change by increasing this by 1 bit for new MAC address
nvram show | grep addr # shows all addresses; make sure my new MAC address is not used by anything
#SET
nvram set wan_hwaddr=aa:bb:cc:dd:ee:ff # new mac address

nvram get wan_hwaddr

#SAVE
nvram commit

#reboot router

I tried executing the both code both in the web browser (under Tools) and using SSH. The settings were not saved after reboot.