I do the gap filling thing all the time - that was pretty much all my first year of feuding consisted of.
As for actual retcons, I've done a couple, yeah:
- Changed the date of an episode in Darkness's backstory from the early 90s to the mid 90s, because I'd just done the maths wrong and there wasn't enough time for certain events to have happened.
- Magnus and I did an in-story retcon whereby it turned out Selenia
wasn't possessing Natalie Portman's body... I persuaded him that it was silly and fanboyish, so Dante went back in time one feud and undid it. We also made fun of the horrible continuity in the feud where it first happened (which was an 8-man clusterfuck) by saying it was a result of the time travel messing with history and confusing everyone. No one knew quite what they were remembering, it turns out
- The thing about Darkness transforming into a big super-powered angel sometimes has been quietly swept under the rug because I don't like it any more. In the past he used to 'become' Azrael in order to fight some really powerful enemies, but I've since had him beat up Lucifer without needing to do anything fancy, and I can't imagine we'll see any more angelic manifestations from him.
- The Hellfire Club as described in the NHFC's bio and referred to in our feuds doesn't actually have much in common with the real Hellfire Club that existed in 411fed circa 2003 (right...?). We've added certain motivations and a level of coherency that didn't really exist in hindsight to fit with the stories being told now.
- There's loads of random little things that happened in shows that never panned out, or didn't fit with what came after, that I mostly ignore. Like that 'trick stable' idea that happened soon after Infinity formed, where Creative wrote a segment that made it look like Darkness was leading a reformed Dynasty. Since nothing ever came of all the surprise returns at the time, I've never acknowledged it in feuding.