After reading through the forums here, I recalled I had a one of the free digital tuner box's from my provider (that they had to provide when they went all digital), that can be used with older hardware.
I have a Tivo Series 2, and once I grabbed the little digital tuner box, and got that between the cable in and the Tivo box, it works great.
I only point this out because it required the Tivo IR extenders in order for the Tivo remote to change channels on the digital tuner. As I've had my Tivo for at least 10 years now, I was afraid I had long throw the extender out, but was able to find it. I tried another IR extender I had and it didn't work. So if you're looking to get some final mileage out of an old Tivo, you will likely need the IR extender that came with the original Tivo.
As others have pointed out, recording is in standard def, but if you don't mind that, you should be good to go.