Is there a source for canon changes and retconcs in Warhammer 40K?

Question

Is there a definitive list of canon changes in Warhammer 40K? I've been reading some older source material, and some more recent codexes, and it is hard to keep track of what has been changed at times, particularly with races that have changed a lot over time. I've tried looking through the wikis, but most of the articles I can find only list current lore, without much tracking of changes over time.

Answer 1

Everything is canon, but not everything is true.

If you are interested what is true in the game, then you read the latest codex. If you are interested what is true in the whole universe then... you decide - some books after all might be a heresy, xeno manipulation or plain Imperial propaganda:

"[...] is the job of authors and games developers to illuminate and inspire, not to dictate. Perhaps you disagree with the portrayal of a certain faction, or a facet of their society doesn’t make sense in your version of the world. You may not like the answers presented, but in asking the question you can come up with a solution that matches your vision. As long as certain central themes and principles remain, you can pick and choose which parts you like and dislike."

Gav Thorpe about canon in 40k.

I think the real problem for me, and I speak for no other, is that the topic as a "big question" doesn't matter. It's all as true as everything else, and all just as false/half-remembered/sort-of-true. The answer you are seeking is "Yes and no" or perhaps "Sometimes". And for me, that's the end of it. Now, ask us some specifics, eg can Black Templars spit acid and we can answer that one, and many others. But again note thet answer may well be "sometimes" or "it varies" or "depends". But is it all true? Yes and no. Even though some of it is plainly contradictory? Yes and no. Do we deliberately contradict, retell with differences? Yes we do. Is the newer the stuff the truer it is? Yes and no. In some cases is it true that the older stuff is the truest? Yes and no. Maybe and sometimes. Depends and it varies. It's a decaying universe without GPS and galaxy-wide communication, where precious facts are clung to long after they have been changed out of all recognition. Read A Canticle for Liebowitz by Walter M Miller, about monks toiling to hold onto facts in the aftermath of a nuclear war; that nails it for me. Sorry, too much splurge here. Not meant to sound stroppy. To attempt answer the initial question: What is GW's definition of canon? Perhaps we don't have one. Sometimes and maybe. Or perhaps we do and I'm not telling you."

Marc Gascoigne (overall manager of Black Library prior to 2008)

So back to the original question:

Is there a source for canon changes and retconcs in Warhammer 40K?

If you are looking for the CODEX changes, there are sites aplenty - i.e. here about the Space Marines. If you are after the lore changes - not so much, because the lore changes can be conflicting and there is no one that would tell you that the new lore is the correct one while the old one is not. Places like 1d4chan, Lexicanum or Fandom wiki often display information about the lore changes, but they are fan-controlled, so don't expect that those updates will happen often. The Warhammer Community usually has the latest news regarding anything Warhammer related (including Warhammer Fantasty, 40k, Age of Sigmar - both about the lore as well as the rules).