It's not possible without a fork in consensus.

Cool... That's what I thought. They can just go fork themselves.

The real question is “why is it not possible?” The answer is NOT “because it’s hard coded every 210,000 blocks.” No. Not even close. The real answer is “Because I say so…and I can say so because I run my own node…and my wallet connects to my node and I will not accept payment that doesn’t follow all the rules.” Back then, neither miners nor businesses thought users had any role. Devs knew better. Most devs at least.