'Retcon' is short for 'retroactive continuity' -- a retroactive alteration to events that officially 'happened' in a story. This alteration can be done by simply expanding upon backstory that was left vague or unexplained -- like the blood elves' presence in Azeroth. In WC3, it was stated that Kael'thas took the 'remaining survivors' of his people to Outland with him; in Burning Crusade, it was further explained that he took the healthy survivors with him, and blood elves that were too weak or ill to make the journey were left behind. This makes perfect sense -- you wouldn't want to take sick soldiers into battle, and the soldiers that Kael'thas had with him during the Dalaran campaign in WC3 were the strongest of what was left after Arthas decimated Silvermoon. No big deal.

Another way this alteration can be performed is by rewriting historical facts and stating the world has always been this way. The biggest example of this is the origins of the draenei: in the original history of the world, as documented by the book 'Sargeras and the Betrayal,' the following is stated:

The eredar, an insidious race of devilish sorcerers, used their warlock magics to invade and enslave a number of worlds. The indigenous races of those worlds were mutated by the eredar's malevolent powers and turned into demons themselves. Though Sargeras' nearly limitless powers were more than enough to defeat the vile eredar, he was greatly troubled by the creatures' corruption and all-consuming evil. Incapable of fathoming such depravity, the great Titan began to slip into a brooding depression. Despite his growing unease, Sargeras rid the universe of the warlocks by trapping them within a corner of the Twisting Nether.

During the development of TBC, when the Draenei were announced as the new Alliance race, the following was published on the official Burning Crusade website:

Nearly twenty-five thousand years ago, the eredar race arose on the world of Argus. They were extremely intelligent and had a natural affinity for magic in all its myriad forms. Using their gifts, they developed a vast and wondrous society. Unfortunately the eredar's accomplishments caught the attention of Sargeras, the Destroyer of Worlds. He had already begun his Burning Crusade to eradicate all life from the cosmos, and he believed that the brilliant eredar would be pivotal in leading the vast demonic army he was gathering. Thus, he contacted the eredar's three most prominent leaders: Kil'jaeden, Archimonde, and Velen. In exchange for the loyalty of the eredar race, Sargeras offered untold power and knowledge.

This created an absolute uproar, as it appeared that Metzen himself had forgotten the origins of the world he created, an impression that was verified by Metzen's apology regarding the matter:

Right... To be totally up-front with you guys, it's my bad, straight up. The obvious lore contradiction with Sargeras and his encounter with the eredar was clearly documented in the Warcraft III manual. I wrote those bits about four years ago, and to be totally honest, I simply forgot. Genius, right? With my excitement to get the draenei up to speed and root them more firmly in the setting, I forgot to do my homework and go back over my earlier writing. I can assure you, no one's more crushed about this mistake than I am. I've spent the last few days kicking my own ass over this one. Sucks to fail. It may not always be evident, but we take this story stuff really seriously at Blizzard. It's been one of my personal missions at this company to maintain a high level of integrity throughout the Warcraft game setting (all of them, actually) and I think we've done a pretty decent job of upholding the continuity over the years.

While this is an alteration made by rewriting historical facts, it can actually be filed under 'expanding existing history' by looking at it from another perspective. While the denizens of Azeroth were under one impression that historians noted and documented in books like Sargeras and the Betrayal, at the time those books were written, they had no idea the draenei even existed, save for the few mutated, largely insane Lost Ones that roamed the Swamp of Sorrows and the Blasted Lands. Upon actually meeting the draenei, it was obvious that the historians simply got their facts wrong -- easy to do when you weren't actually there.

I'm going to evoke Godwin's Law here and make an analogy: it could be looked at as having history books that stated that the actions against the Jewish people during the Holocaust were warranted, and the Jewish people being presented as being somehow wrong or evil, forcing Hitler to take the actions he took during the course of World War II. Then suddenly, a large group of survivors from the concentration camps are introduced who are not only not evil, but have a very, very different story regarding the events that took place. Considering all you had to work with at first were stories written down by historians that weren't even present when those events happened, you cannot really argue the events of history with people that were actually there when those events took place.
Think of the Warcraft story as a book. Instead of being a book like Harry Potter; in which the universe has been defined, events have already happened and everything is already established as truth, you are reading a novel that is literally being written page by page as you go along. The events of present day can and will alter the perceptions of the past, and events that occurred in the past can change at any given moment based on what's happening currently. What you can take from all of these changes is this: it is extremely difficult to keep an ongoing story fresh and entertaining without writing yourself into a hole that you have to pull yourself out of. That's why changes are necessary.

With canon being so iffy, timeline shifts negating current lore, retcons happening periodically and entire previous parts of history being rewritten all the time, what's a lore researcher supposed to do? Well, you can either throw up your hands and say 'to hell with it,' which is no fun at all, or you can simply enjoy the story as it's being written, without getting too attached to events or characters in the way they've been presented.

The passion behind the reactions to timeline shifts and continuity changes is a testament to exactly how convoluted and entertaining the Warcraft story is; and while at times it seems like the story can get out of control, it's obvious from fan reactions that the story behind Warcraft is a good one. People like myself that document lore have to keep in mind everything that's been presented above, and documenting events and history according to lore is a lot more difficult than most people realize. The most important thing for beginners in lore research to remember is this: when it comes to Warcraft history, the best weapon a lore hound can have is an open mind.

Stay tuned for next time, when I talk about speculation, theory, and fanfiction!