No, you can't just drop a new HD in your TiVo and expect it to work. It must be imaged with something like InstantCake or one of the other options before it will work. The image is specific to the make and model of the box.
In my experience TiVo has treated HD upgrades with a wink. However I'd keep the original and put it back in if I had to return it for service just in case.
In my experience TiVo has treated HD upgrades with a wink. However I'd keep the original and put it back in if I had to return it for service just in case.