Trying to pump some life back into this thread a bit
I was trying to think of examples of games, where the sequel has improved upon the original BUT not made the original irrelevant.
I'm not talking story here but gameplay. I mean look at Assassin's Creed 1 and 2. The first one is good, but the second does everything the first one does and more, in a better, more fun way, making 1 basically obsolete. The same goes for 'Dead Rising'; 1 was awesome, but with the addition of combo weapons, less dumb survivors and a much larger area, DR2 totally eclipses the original.
I'm not saying these sequels make the first game worse, but if you think "I fancy a quick play on <game>" you're more likely to pop in the sequel than the original in these cases.
Hence why I put it to you guys if you can think of any sequels which don't make the previous game redundant (note, the sequel has to be good, I'm not looking for games where the sequel is worse
A couple of examples I thought of:
Metal Gear Solid 2 and 3. Both great games, but 3 doesn't stop 2 being fun to play, because they are set in completely different eras and use different tactics to get by.
Resident Evil 4 and all previous Resis. Because of the shift in gameplay, 4 is still as much fun to play as the classic ones.