Fitting that this thread was revived today...

I just called to cancel my old radio and activate the new one in my new car. I was on the phone less than 10 minutes, and my old radio was cancelled and my new one was activated. They even gave me an extra 3 months free because I paid for 3 years up front. Easiest customer service encounter I think that I have ever had.

This was from XM by the way.