Do you need to transfer the shows to the new tivo, or just have them available for viewing?
I would use pyTivo (I suppose desktop would work, but it is not nearly as powerful) and save them onto a PC/server for pulling to the TiVo when you want to view.
Depending upon your cableco and how they treat copy protection, you may/may not be able to transfer the shows at all.
I'm sure someone will note streambaby for season passes, but I don't use it or know its capabilities. Try searching that term. I have not had problems with the online season pass manager.
In the alternative:
If your HD is not lifetime, try to get it for $99 when you call to cancel, then just use it for storage or as needed. This only works if you have the ability to transfer shows.