Blizzard had this to say on the forums regarding what will happen to raiding gear sets in the expansion.

There has been quite a bit of discussion on the boards lately concerning how current end-game gear will be replaced upon leveling in the Burning Crusade and we thought that many of you would appreciate some clarification. The basic idea is that players who have recently obtained level 60 will gain access to immediate upgrades from expansion quests and creatures. Our exact estimates as to when tier 3 gear will become outdated have shifted back and forth somewhat as we continue to tune the expansion, however not dramatically so. While we do anticipate players wearing full tier 3 gear to find some upgrades/sidegrades from new endgame dungeons, the big draw will be in Karazahn and higher.

Please keep in mind that this information is simply meant to provide everyone with some insight into this process, and much like any aspect of game play that's still in development, it's subject to change.