Thats happened before. Once Riot said in the patch notes they sent out a Nerf on a champ (I forget which but it was some basic number stuff) but the thing is due to a mixup, that never went live. I think it even had to be hotfixed in
Yet that whole week it wasnt up EVERYONE PLAYING TE CHAMP BITCHED THEY WERE USELESS! Despite no actual changes. This is probably (theory) due to the fact that 99% of the playerbase dont actually know shit about the game. Obvious stuff like a move changing how it works or cooldowns being longer sure, but most folks cant tell me any stats on even their fave champ (by stats i mean stuff like base attack, dmg growth, health growth, etc) so when thats all that gets changed the playerbase just goes "Oh that number is lower/higher so they must be bad/good now"
Of course over time some players feel the changes more, and obvious fact is obvious but the 1% that always can tell are high elo/pro players and developers alike because duh, but yeah for the most part league player brains just see a number and adjust their expectancy of the champ based on if it's being called a "Buff" or "Nerf" so really, this would be very easy to do.
PS - Personally since we can have URF anytime now, i think April Fools should just release the silly skins and have some kinda like IP party week thing, BUT you only get the bonus playing ARAM or Twisted Treeline! (modes under normal situations few people touch lol)