one of the techies here, maybe Sub, Bass or Moist made a comment on how to tell if the analog to digital conversion had been done. Something about the shape of the antenna. Do a search here. My 2003 was apparently converted before I bought it. Bass is probably right, your '04 is already digital.
The old analog system had the antenna loading coil in the middle of the short whip. The digital system - different frequency - has the loading coil at the bottom 1/3 of the whip.
Ask a dealer to run your VIN and see if the change kit is available, if necessary. OnStar will advise them, they order the upgrade, you get a phone call when the kit comes in. Service appointment, about an hour and a half including at least a year subscription ($$$.$$ plus maybe 200 minutes - $$.$$). The install is either dirt cheap or free, but they rape you on service and minutes.
Otherwise you'll need to press the OnStar button and speak with a rep about starting your own account.
A couple of years ago, our auto insurer, State Farm, was offering discounts on insurance to people who have OnStar systems. The idea was that mileage and maybe a few other driving parameters could be read via the OnStar system, telling State Farm about your driving habits. This would allow them to give lower rates to better drivers. Our '04 DeVille has the analog OnStar. The dealer told me that they could modify the system to digital and give us a one-year subscription to OnStar for $99. When we did the math, it turned out that our potential discount on the insurance did not equal the cost of the OnStar subscription, so we passed and the car still has the (inoperative) analog system.
I recently removed the antenna from the rear window. I figured it would save time for some vandal who wanted to break it off of the "rich guy's car".