You can purchase only HDTV channels, however you may be subject to a $5.00 access fee each month if you do not have a basic package such as the America's Top 60.
You should be able to order it ($5 access fee applies of course). If you have an HD DVR (942) your DVR fee will be $10 instead of $5. Technically you would be paying an extra $10 instead of $5 as a result.