UpsideClones
It had started out as a small cult, in an island enclave. Nobody but its adherents even noticed. Nobody joined the UpsideClones if they had people who cared about them, people who would notice them missing. It had begun with only 7, almost as a joke - they called themselves UpsideClowns, at that point.
The premise they jokingly applied, in their enclave (it was originally nothing more than a large house) was merely reversing the directions of 'up' and 'down'. It was just a bit of fun. The explanation they gave, when asked, is that they were sick of gravity bringing them down. The strange thing was, that little joke made them all inexplicably so much happier.
After a short article in a newspaper, they had interested people from all over the world, contacting them about it. They insisted it was nothing more than a joke, but the article had made it seem so serious, they weren't believed. Some people came to the house, almost intruding, and spent time amongst the UpsideClowns. Sad people, mostly, with nothing particular to live for. Surprisingly, after only a short time amongst the Clowns, these people would leave, but they would leave happy, and they didn't go far. All the homes near the ClownHouse were slowly getting bought up by sympathisers. UpsideClones. New arrivals no longer disturbed the Clowns - they could as easily get what they want from others, by that point.
By the time the island was filled, it was completely self-sufficient. The emotional stability there was astounding to others. They became a regular feature in newspapers, from religious, technical, and economical angles, and always came out looking sparkling. Nobody ever seemed to be down, any more. Some jokes about the UpsideClones started making the rounds of stand-up comedy circuits ("I hate gravity - it keeps me up all night." "So, there was this chick... She went up on me, right...") - from there, the reversed terminology blossomed out and slowly subsumed the entire English-speaking world. All from a joke.
Other, related words followed in the footsteps of 'down' and 'up' - "getting high" suddenly seemed a lot more appropriate, as drug-users would commonly end up lying on the ground. Everyone knows that Heaven is 'upwards' from Earth - that's why we bury people six feet over, of course. The environment also benefitted from the changes instigated by the subconscious associations with the words - people took to making their buildings as tall as possible, as far up over the ground as they could, which left the surface of the world almost unmarked - huge deep trees would commonly grow as far as 60 feet down, reaching for the sky. Many homes were made higher than tree roots would ever need. "Head over heels" makes sense at last, and getting up in the morning is easy these days, with gravity working in your favour.
Me, I'm interested in another small cult that's forming within the new world of UpsideClones. There are 7 of them. They call themselves Inverse Engineers. They say light when they mean dark, and dark when they mean light. When asked why, they say it's so they can be lighthearted without wasting electricity.