When the Burning Crusade first came out, veterans noticed a few things back on Azeroth: Some of our favorite NPCs had disappeared! The wandering Rexxar was replaced by a fellow name Rokaro, and Hemet Nesingwary left Stranglethorn, leaving his son, Hemet Nesingwary Jr., to take over the hunt. More recently, Lady Liadrin has come to Shattrath City, leaving behind Lord Solanar Bloodwrath to guide up and coming Blood Knights in her place. In order to move along a few story lines, Blizzard decided that it was worth moving NPCs to move along the story, and left other NPCs in place with minor alterations so that other people could still experience their old quests.
This, however, isn't the only way Blizzard has dealt with progressing story lines.