When you connect your new modem and try to get online, you will immediately be redirected to an activation page to register your modem. You need to enter your account number (16 digits, starts with 8383) and a registration code (last 9 of your account number). From that point, click on swap device and the page should pull your 6120's MAC address automatically. If it asks for a tech number, enter 0000. The modem will reset and you should be good to go.
After that is done, you can drop the Scientific Atlanta modem off at the local office and the modem rental charge will be removed from your account.
Let me know if you need any further clarification.