It is no secret that I am a fan of most editions of D&D (and many games in general). Since I began back in 1979 I have played every edition of *D&D there is and have found something to enjoy in all of them.
Since I have been playing for so long, I have also had campaigns that have lasted years. Sometimes these campaigns span multiple editions. For example, my kids started with characters in 3rd edition, then those characters have kids that were started in 1st Edition and then we all moved to 5th edition. With the occasional side step into Basic or OSR games for fun. I have used different editions of the game for flashbacks, dream-sequences and general out-of-body experiences.
But looking at the larger picture of a longer narrative have you considered the actual rule changes to part and parcel of what is going on in the world? Obviously, if you only play one edition this will not mean much to you or if your games have no continuity between editions. But I have characters that started in Holmes Basic and they have descendants in my current 5e game. Usually, it is one generation per edition, but how can I explain it when a cleric only has a mace a weapon and no spells till 2nd level when his grandson, who is also a cleric of the same god can wield a sword in some cases and his son can cast minor spells at will?
Some things I did work into a large narrative.
When I went from Basic to First Ed I explained the Class/Race Split by saying that elves in my original lands preferred to become fighter/magic-users due to tradition, but elves elsewhere in the world would choose other classes.
Going from First to Second had the biggest hurdle regarding demons. First ed had them, second ed originally did not. So since I had just done a huge war to finish off my "high school" games before college I just said that the war had blocked all demons from coming back into the world.
Second to third was a longer time span of inactivity for me, but the big issue was the birth of Sorcerers; people with spontaneous magic in their blood. Is this a remnant of the re-opening of the demon gates? Maybe. Hmm....I think I see and adventure idea!
Fourth has a slew of problems. Mostly though the change in the nature of magic. I have regarded this as an odd conjunction of the planes; something that altered the Cosmology. Again, sounds like a cool thing to play out one day.
Fifth then is the return to the way things were before...with some things changed permanently.
I know there are some "in-story" and "in-universe" explanations for these changes in a lot of the Forgotten Realms material. I will have to check these out someday and see if they track with my own ideas.
What have you done?