I have Comcast and have been trying to figure out what it all means.
The FCC forced the cable industry to allow a "cable card" to decode signals. Some DVRs have cable card built in, some have a slot to install them. Comcast will probably lease you a DVR, or I believe Tvio can be used.
Some cable cards can decode multiple channels at the same time (so a DVR can record multiple channels). I don't have one - probably can record and pass through different channels.
Some TVs also have a cable card slot. I have read the FCC set a max fee of $2 for a cable card. Here the first one is free. A cable card in the TV and a converter ahead of the VCR is the equivalent of 2 converters.
From what I have read "Tru2way" capability built into a TV allows watching pay-per-view, which Comcast would like you to do. As far as I know, if your TV does not have Tru2way you can still use a cable card and just not have the interactive functions.
The only way around having some kind of decoder for the upper channels is if one of the channels you want is direct broadcast and use an antenna. Those channels are probably available on Comcast as the low channels unencoded anyway.
The reason for the digital changeover is, presumably, when Comcast gets rid of the old "analog" channels and switches to "digital" they can add more channels. This is partly because digital takes less bandwidth. They can also compress digital to reduce bandwidth (which degrades the signal).
I have found it real annoying how hard it is to get good information on all of this. (Anyone have any good sources?)