Needs to be pointed out that Fr wasn't the only one to have lore explanations for edition changes. Die Vecna Die was in fact relevant to all settings (although the action only took place in Ravenloft, Greyhawk and Planescape) and Greyhawk had its own 1E -> 2E adventure, Fate of Istus. Assassins and monks would actually change into other classes in the conclusion of the adventure, a change that only makes sense if you remember those classes disappeared in 2nd edition. Mystara never got one because it stayed in Basic D&D until the end of that line and by the time it transitioned to 2nd Edition it was TSR's final days anyway. Dragonlance didn't have a transition adventure either, as I recall, but it had a sort of rocky, zombie-like existence in late 1st Edition and 2nd Edition. Of the 2nd edition settings, Spelljammer was already dead by the time 3rd edition rolled around, as was Birthright and Dark Sun. Al Qadim and Kara Tur were already folded up into the Forgotten Realms.
Interestingly the event in Dragonlance that might seem to be mandated by edition changes, the "Summer of Chaos", was not about transitioning the world from 2nd edition to SAGA. In fact, the novel was written first, and the game developers were left to try and scramble and build up a playable setting from the ashes. (It was a testament to some of the good design there that they did, even if people rejected it because it wasn't what they were used to.) Similarly, the War of Souls was more a Sundering-style reset to the world rather than an "explanation" of going from SAGA to 3rd Edition D&D, since the new edition of the game had been out for a few years anyway by then.
