When a player drops from an official tournament does anything happen to change the results of earlier battles?

Consider:
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Player A plays the first 2 of 3 scheduled rounds:

Round 1:
A defeats B, both A and B score some points
C defeats D, both C and D score some points

Round 2:
A loses to C (both scoring points)
D loses to B (both scoring points)

Before (or during) round 3, A decides to drop from the event.
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presumably the pairings for round three were going to be: (B v. C) and (A v. D) ...D will get a bye and a final record of 1 win, 2 losses)...Player C is either going to be 2-1 or 3-0, but what will the record be for player B?

99% of players/judges would let the round 1 results stand, so that player B is either 2-1 (with a round 3 win against C) or 1-2 (with a round 3 loss to C)...but this leads to confusion over who gets 2nd place...since the way points for the bye are determined will not match what happened in th eearlier rounds, making it very unfair for at least one of the remaining players?

But if the rules say that player A is considered to 'not have participated in the event', shouldn't the results of rounds 1 and 2 be revised so that:

Round 1 Player B gets a bye/win, Round 2 Player C gets a bye/win (just as player D gets a bye/win for Round 3)? This makes round 3 a battle of two 2-0 players.

Technically, to be truly fair, you'd have to call any game played against player A a bye, otherwise Player A's third opponent is getting an advantage that his previous two opponents did not. But man, I gotta tell you that this can really screw up a tournament...

That's a slippery slope, to be sure. Unfortunately, substituting early results with a bye could have affected pairing in subsequent rounds that may have already happened. For instance, the round 2 pairings could have been different if player A's first round game were scored a bye instead of the actual result. This assumes you use Swiss pairing, of course. On the other hand, giving a bye only to the later opponents can present an unfair advantage as GBoy mentioned. I would suggest the following.

If a player must leave during a tournament, immediately move that player to the bottom of the rankings for the duration of the tournament. When the second to last ranked player draws the absent player, he/she gets an automatic bye.

Anyone who draws this player after he/she has left is near the bottom of the heap anyway, so this shouldn't adversely affect the results. The leaving player may have been near the top of the rankings, which could have resulted in a prize if more than one ranked finisher is awarded a prize. In this case, I say you should stick around for the whole tournament in order to qualify for a prize anyhow.