I did. I went from working full-time in the tech industry to working part-time in the games industry. Similar work, different environment and culture. Way less money, way more happiness.
The thing is, though, that "downgrading" to part-time or to something more fun still lands you in a work environment where you have obligations, bosses, coworkers, deadlines, politics, and other work stressors. I've taken months and even years off during my career, and there's an expansiveness of spirit that you get to after a few months of not being accountable to anyone that just doesn't set in even in fun, part-time work. You're still basically in an institutionalized mindset, even if you enjoy it more. So it doesn't smooth the transition as much as you might think.
I enjoy what I do, and I like that it gets me up and moving each day. I've got it pretty cushy, and the constant problem-solving is good for my brain. But I know when I do retire, it will be as much of a shock to the system as if I'd been working full-time.
Personally, I think a better way to ease into FIRE would be freelance work where you take chunks of time off. I used to freelance fall through spring and then take summers off. That more closely approximates FIRE. But it also brings more uncertainty, more hustling to find work, fewer employment benefits, and, often, much lower pay.