My question is: Is it really necessary to make big noises about retcons in your own fanfiction series? I mean, if you're just world-building and you haven't written a single story in your series that actually uses any of the characters and elements you established for it, can you really call it a "retcon" if you change things that contradict earlier information of those same elements and characters? I have been actively writing the EMPATH: The Luckiest Smurf story series since the 1990s, and I don't really make a big deal about any changes in the series since I already have an established history for the series, and I usually consider my stories to be in a "perpetual beta" state, meaning they are always subject to change whenever I feel they can be enhanced or improved with new or modified material.
Let me make this clear: I'm not saying that you shouldn't inform the reader when you're going to make major changes in your fanfiction story series that's going to result in major conflicts in the overall storyline. If you know the reader is going to get confused when he originally reads something that was written to go one way and you change it later to make it go another way, I think it's fair to keep the reader informed. I'm just more concerned about "making big news about minor things" unless you personally feel it's important to let the reader know about the minor things being changed in your story series.