Well, the answer to keeping your recordings is yes and no. If you were simply expanding the drive, it would be possible.

If you were interested in the hacks for video extraction, then it wouldn't be a possibility. An important part of the hack is turning the scrambling off, and once you do that, the tivo won't be able to make sense of recordings previously saved when the scrambling was on. (It looks like the ability to watch scrambled shows isn't impossible, but it looks a little complicated.)