You can change the serial number of the Tivo using the crypto command. Have a search on the Tivo hacking FAQs and forums. Only do it if the other Tivo is dead however as they do check to see if multiple units are downloading guide data using the same serial number.
Can't you get the Tivo repaired? From what I remember, they'll transfer the subscription only if your old one is broken. It should be cheaper than buying a new one... (Unless of course you've already got one)
It's not hard to upload your own guide data into the Tivo. Took me a day or so to work out how to generate the correct data. I only did it to see how it worked as I've got a normal subscription anyway. Tivo is really useful IMO and I don't want to see them go down the tubes.

- Trevor