After I saw this article, I did some checking on Comcast's X1 box and found the page:
www.comcast.com/x1. Since I've got two DVRs and they haven't upgraded the technology in nearly 10 years, I called to see if I could upgrade. In case anybody's interested, here are the details:
X1 HD DVR - $20/month, $70 installation fee
X1 HD Box - $9/month, $30 installation fee
I balked at the installation fee, so they knocked it down to $25.50 and $5.50 respectively. Turns out I didn't need two DVRs anymore because the standard HD box can access the content on the HD DVR, which is cool. Since I was paying about $36/month for both DVRs before, I'm now saving about $9/month.