This is a non-exhaustive list of retcons for Warcraft. Most of these are just ones that I’m aware of or which I personally noticed.
They are categorized by the origin of the retcon. So…games, expansions, books, etc.
If I’m not sure exactly where the retcon came from, but am sure that something was retconned, it will be placed in ‘unsorted’ for the time being.

A pattern I’ve observed is that the books and manuals tend to be treated with less respect than the games (this isn’t universal, as the RTS games are also happily ignored).

This question has been bugging me for a bit, but I’ll leave most of that for the coming retcon article.
Generally speaking, retcons are a bad thing, but can be good in a few cases. The main exception in which retcons are acceptable is if they’re fixing something broken.
But, well, I can say a few things conclusively. A ‘pointless retcon’ - a retcon that has extremely minimal impact - is bad automatically. They’re bad because they are unnecessary considering what they’re changing is tiny and, as stated before, the resulting impact is tiny.
Another thing to note with retcons is that they should be incorporated into the original lore properly. So, on the surface, a retcon should not be outwardly perceivable as a retcon. The best WoW-related example is probably the Sea Witch.

This is a non-exhaustive list of retcons, but if you are aware of some and would like me to include them…I suggest you suggest I do that.
Though, and this is kinda important, you should include some proof that it is a retcon. Like what it’s overwriting and how it was overwritten.

One of the big reasons I’m doing this is because the Warcraft Wiki doesn’t have a page like this. The closest they do have is a ‘retcon speculation’ page which, obviously, implies something else: these are speculated retcons and not actual retcons.
The second-closest are all the retcons they mention on the Word of Warcraft Chronicle^A) pages, but they otherwise seem to just…not acknowledge that retcons are a thing.
This is one of the major contributions to Memory Loss.
I’m half-suspecting this is intentional on the part of the Warcraft Wiki, though I can’t think of any motive why besides defending Blizzard’s story decisions for no good reason.

I’ll gradually adapt and refer to the retcons listed on the following pages:

• World of Warcraft: Ultimate Visual Guide, Updated and Expanded
• World of Warcraft: Ultimate Visual Guide
• World of Warcraft: Chronicle Volume 1
• World of Warcraft: Chronicle Volume 2
• World of Warcraft: Chronicle Volume 3
• World of Warcraft: Chronicle Volume 4

I’m not 100% sure on the details, but I think it was Day of the Dragon that retconned Alexstrasza’s status as the Queen of the Dragons.
In Warcraft 2, the dragons were green (or brownish-gray in the cinematics) and, well, Alexstrasza was stated to be their Queen…which, of course, implies all dragons.
The change that occurred made her the Queen of the Red Dragonflight…meaning the color of the dragons was retconned as well.

The Draenei originally had a completely different backstory and were, get this, native to Draenor. After the orcs appeared, they started wiping them out.
By The Frozen Throne there are fewer of them, but they’re still capable of fighting back. Their leader was Akama and, in the Blood Elf campaign, he’s who you ally yourself with.
This was retconned…like…okay, this is one of the more perplexing retcons because there are two possibilities:

• The writers forgot about Warcraft 3.
• The writers intentionally ignored Warcraft 3.

I’m leaning towards…both options at once. They had somehow genuinely forgotten what they originally wrote and, upon it being pointed out by someone…they just ignored it.
This is especially perplexing because the best option in this situation would be ‘wait, we already came up with the draenei? Okay, guess we’ll re-name this new race, no problem!’ but they instead went with the retcon…which would obviously not sit well with the fanbase.

At the end of The Frozen Throne, Illidan and Arthas fight and, at the end of that fight, he gets slashed and is left to bleed out in the snow. This cinematic naturally heavily implies that Illidan died.
However, Illidan later re-appears in WoW as if little has happened and he continues being a thorn in the side of everyone.
What I find interesting about this retcon is that I vaguely remember it being discussed at some point, but has since been entirely forgotten about.
Also, the Warcraft Wiki amusingly claims that it was the original intent to show Illidan dying, but this didn’t make it in due to time constraints. Never have I seen such obvious ‘cope’ in my life.
Also also, when I was about 13 years old, I was playing a custom campaign which continued the Blood Elf story after The Frozen Throne. In the first mission, I witnessed Illidan’s rescue and, consequently, saw his recovery. At the time, I assumed it was bad fanfiction…now I know that, no, it was an adaptation of the post-WC3 story.

This is definitely a retcon and a very stupid one.
Basically, at the end of the expansion, after you defeat the Lich King…Bolvar takes his place on the throne, donning the Helm of Domination and then…Bolvar is the new Lich King. The justification given for why this is done is…that there must always be a Lich King.
However, none of the previous story up to this point indicate that this needed to be the case.
I mean, let’s think about this logically for a second: there was a point in Azeroth’s history where a Lich King didn’t exist…but even assuming if something fundamentally changed, this is a very unusual thing to assume.
Now, this is me fixing the story on behalf of Blizzard, but one way it could have been justified is if a Lich King were needed to control the Scourge because, otherwise, they’d go completely haywire…causing chaos and destruction. Though, again, I’m the one doing this…Blizzard never provides this kind of justification.
Another way this could have been justified is as a form of controlled extermination. While the Horde and Alliance were busy eliminating the Scourge, they could have had Bolvar control it to prevent it from fighting back. This is also something that Blizzard would not have done, because status quo…the undead just kinda have to exist here now despite being against the natural order.
As far as I know, Chris Metzen (yes, really) is the one responsible for this…and he did it just because they thought it would be cool. I don’t think this should have ever been done, though.

The Warcraft 3 Demo’s Orc campaign^B;C) has a ‘Sea Witch’ as the primary antagonist in the second half of the story.
In Warcraft 3, she’s depicted with a Ghost model/portrait/voice. Cataclysm claims that she was a Naga the whole time, technically making this a retcon.
Reforged takes this a step further and just turns her into a Naga Sea Witch, though I’d consider that a separate retcon.
Her being a Naga the whole time is the first positive retcon that I’m listing here and, so far, appears to be the only one.
This is a good retcon because it doesn’t over-write information as much as it elaborates on a mystery…that mystery being ‘what the Fel was the Sea Witch?’
Even her vague warning about a coming dark tide now fits neatly in with the Naga stuff.
However, the Reforged one? I’d say that was probably a negative change all things considered. I think she should have remained a shadowy, mysterious ghostly figure…maybe only slightly hinting at the fact she’s a Naga. Because of this change, the Horde knew about the Naga existing waaaaay before anybody else: they literally met one. Originally, they had no clue what she was or who she was…but now they pretty clearly know she was some strange fish/snake creature. I may also be over-thinking this point, but I really don’t think this was a change for the better.
But otherwise? Yeah, positive. Good job Blizzard.

Zar'jira Sea Witch (Warcraft III)

This is one of those ‘pointless retcons’.
In Warcraft 3 and The Frozen Throne, Tyrande has dark blue hair (navy). In WoW, this was changed to her having teal hair. In a now-deleted tweet, which is cited on the Warcraft wiki, it’s stated that she dyes her hair teal. So we can assume that, sometime before WoW, she started dyeing her hair.
However, in Reforged, her hair color was changed to match her WoW appearance…so teal hair. This means that her dyeing her hair was retconned.
I fully realize that this is a ‘who cares’ situation in the grand scheme of things…but I care because that Night Elf box art^D) was one of little Ninja’s first waifus. Further, I don’t think I liked the ‘hair dyeing’ thing at all and — and I will die on this hill — she looks better with navy hair. Sorry not sorry.
But, like, guys…seriously? You saw the original game and yet you still screwed up?

Tyrande Whisperwind#Notes_and_trivia

Elune is the super mysterious goddess of the Night Elves and, to this day…is somehow still the most mysterious goddess in Warcraft.
The retcon I’m referring to is ‘Azeroth’s only true god’ which is apparently said somewhere in Warcraft 3. This is also referenced in the Warcraft RPG. But, since then, it’s likely been retconned to Hell and back.
My guess is that, originally, Elune was the Light. The Light allows people to heal others and, well, the Moon Wells are the primary means of healing others for the Night Elf faction. Plus the Light’s nature is kinda ambiguous as well (until it’s retconned) so…
That’s a guess though, but Elune as Azeroth’s only god is definitely something that got retconned.

Elune

This may or may not be the victim of a retcon.
In The Frozen Throne, the Naga curse is sorta vague. We aren’t sure exactly what turned the Night Elves (later elves) into Naga, though it is definitely related to them being swept under the sea…
The RPG asserts that the curse is related to the Well of Eternity, which also sank. The Naga Anomalies are directly related to that, as those who were exposed to the Well’s energy more than others.
Somewhere in WoW, though I’m not sure where, the curse was (possibly) changed to being the machinations of N’zoth, an Old God.
The reason I’m not really sure about this is because it was ambiguous to begin with, though I’m leaning very hard on this being a retcon.
Oh yeah, I’m also not sure if it’s explicitly referred to as a ‘curse’ in Warcraft 3. Though it’s not like the Naga care.