All the Tivos, like all the Replays, can be upgraded using straightforward ultitiles available on the web.
I have both Replay and Tivo at home. The primary advantages of Replay, IMHO, are these three:
1) Commercial skip button on remote. Great when it works (probably 90% of the shows).
2) Easy to record from line-in. (So, for example, you can play a DVD and record it onto the Replay for
future viewing).
3) Shows are organized by Show name. (E.g. navigate down to Sopranos and you then get a list of all
the Sopranos you've recorded). On the Tivo, shows are listed in a flat list by the time they were recorded.
4) "My Replay", the Web site where you can program your Replay.
The advantages of the Tivo:
1) You can prioritize all the shows you want to record. If there are conflicts (both shows on the same time), the
one with the higher priority gets recorded.
2) Navigation/controls are more responsive
3) At the lowest quality recording setting, you can record more on the Tivo, in the same amount of space, than
on the Replay.
4) If I had to bet, I'd bet that Tivo (the company) will outlast Sonic Blue (who owns Replay).
Kwad