Encounters are usually written in advance and balanced against the makeup and the abilities of the party. Sometimes though the party makeup when writing such an encounter in advance and when actully at the table differs: either because a player could not show up and called in without the DM being able to re-write the encounter, or because of character death.
Now, most parties won't retreat when facing sudden character death and keep on with the (n-1) of them. But the encounters will be scaled to a full-sized party, and as such be more difficult. When one player is down the encounter could still be managed, but when there are significant losses (four or less players taking on a six player encounter) things can go bad fast. In some cases it might be logical for the players to retreat, raise the dead/get new party members and plan for a second attack, but this is not always the best idea because otherwise Bad Things may happen. So how do you, if at all, retool your encounters?
Of course, encounters consisting of gangs of goons could be cut down to match the party, and you might be able to de-level a big monster a bit or replace it with something else. But I'm not too fond of this idea because when you have to fight plot-important enemies such as a group of six villains who are the dark equivalents of the party having some not be there for the big fight is odd, and it would be strange if the vampire villain Count Bloodsuckula got his hands on the Artifact of Significant Badassery and uses its magic to become more powerful... only to be at a lower level than the last time the party faced him because the Barbarian and the Druid got munched on by Wights a few rooms back.
A solution would be to somehow introduce new characters the players build in advance or build them as temporary NPCs to join the party... but if I go through that effort I might as well come up with retooled encounters in advance. So how do you handle the retooling of your encounters?