Well im not gonna touch the Tivo / Replay who's better thing but i have had a serious one Tivo for about 5yrs? and a Series 2 for about a year and it began doing the Hanging on video thing as well, I did a little research and found a web site called weaknees they specialize in selling and replacing hard drives in Tivo, Replay & MIcr** Ultimate boxes. http://www.weaknees.com/replaytv.php
It wasn't to hard! although im not to sure about the Replay boxes? but if you not comfortable with the process they have a great service that will do the replacement for you.
As far as the lifetime service, if the Replay boxes work like the Tivos the "CODE" will be on a chip on the motherboard and will not be affected by the hard drive swap!
At least thats how it was on my Tivo so it didn't affect my lifetime service!
There are so many articles about both Tivos & Replays that i really can't give you info on where to look but i will tell you about my two Tivo's series one & 2 and you can compare them to you Replay.
I have to Tivos, and old Series one the one i replacet the hard drive in (Phillips) and a new Series 2 (Tivo branded)
The Series 2 is the most likely Tivo you would encounter in a store.
Box to Box communications Series 1 no / series 2 yes
Net work transfer of shows to pc or other tivo box in your net work S-1 no / S-2 yes
Internet program data download S-1 no (with out modification) S-2 yes
Scheduling of programs to record from the internet S-1 no / S-2 YES
Season pass program S-1 yes / S-2 Yes
Live Tv pause Yes on both
those are the very basics of the boxes although i haven't had a lot of experience with Replays from what i have seen they seem to work in a similar fashion.
hope this helps a little