This whole thing is pretty outrageous...
One tuner on my Premiere died this week, after only a few years of use. This was pretty upsetting itself, and after troubleshooting it with TiVo Tech Support, I mentioned that instead of paying for a replacement, I'd use my old TiVo HD for now. They told me this was fine.
So after reviewing the article "How to Transfer TiVo Service to a New TiVo DVR" on TiVo's support site (which notes it's fine to transfer service to a TiVo HD), I purchased a new Hard Drive and installed it in my old TiVo HD.
After getting everything setup, I tried to do the service transfer online, and got an error message. After waiting 45 minutes on hold to do a phone activation, I was told that because of cable companies switching to MPEG-4, the TiVo HD can no longer be activated.
So instead of taking my money and making me grumble at the cable company as they convert my channels to MPEG-4, you make me hate you.
The TiVo HD is hooked up right now, with the CableCard installed, and working flawlessly. But it's worthless, because TiVo refuses to supply it with guide data, even though they can.