There's something to be said about the Pokemon mythos and lore, or the world in a scientific view. We've really reached a point where retconning some of the older Pokemon would be nonsensical. It was fine when they did it for Magneton and Magnemite, but that time has long since passed. Older Pokemon have become their own personalities.
I would explain it with a story, in a quest chain. After an eclipse, about two dozen already known Pokemon changed type into a totally new type, and new Pokemon appeared. The player finds out that the Pokemon draw their power from a Legendary Light-type Pokemon that was imprisoned by a seal that could only be broken by a solar eclipse.
It's quite a bit more than we got from Magnemite and Magneton.
Most of the Pokemon I picked out are superfluous normal-types that hardly anyone uses. I think they need something to make them more than just "the cutesy normal-types of which only Blissey is useful". When was the last time you heard someone say "Wigglytuff/Clefable is a great Pokemon for battling! It's type is so unique and useful!"?
Anyways, only Togepi, Togetic, and Togekiss would have to have their properties totally changed.
All other Pokemon on that list would simply get a new secondary type.
The fairy egg group Pokemon to me, just don't gel with the other Normal-type Pokemon. Who are mostly realistic animals with small deviations instead of humanoid embodiments of cuteness, light, and kindness. They're a thematic sore thumb.