Question about changing MAC address

I don't want to use someone else's MAC address. What I'd like to do is generate a new unused MAC address that I can use with Comcast. I suppose it may get issued by the cable modem manufacturer eventually, but until it does, I'd like to use it. The reason I want to do this is because I've had to replace several cable modems over the years and I try to buy them cheap on Ebay. Sometimes Comcast won't let me use a modem I buy because the previous owner owed money. My preference for a modem that will let me generate a unique useable MAC address would be a SB5100. Please let me know if this would be possible using a jtag cable or some other way. Thanks