Read your manual or call comcast to see if the box requires a Dual Link cable. The large majority of things don't require Dual link and if that's the case then those cables will work just fine.
That said, with most companies you can often exchange you cable box for a newer model. So if one if available it will likley have HDMI and seeing as how you typically lease the equipment from the cable company if doesn't cost you much (if anything).