What you are trying to do is possible. However, it cannot be done from a remote location. The reason is because the Slingbox will need to be reset to factory settings, in order to configure to the cable modem. This can only be done by holding down the reset button on the Slingbox.
Also, Watch on Slingbox.com will not allow you to configure a Slingbox from a remote location, unless you have full access to a computer on the same network as the Slingbox.
Below is a link to an article that explains how to reset your Slingbox to factory settings:
Hope this helps!
The Sling Moderation Team