Having done MANY of the upgrades, repairs, changes etc with tivos and directivos, I can tell you it's not hard to do. There's plenty of sites that have step by step instructions.
Your best bet? Copy your tivo image (you should do that for backup anyway), then just put in one big hard drive in there, and expand your image onto that drive. You'll have to boot into linux to do it, but like I said, there are plenty of step by step guides. Very simple, and you'll love the result.
Nothing wrong with the weaknees product by the way, for those not comfortable messing with linux commands, hooking up drives, creating partitions and that kind of stuff, the weaknees stuff is fine.