Careful use of a dremel will allow you to remove that ring -- cut down almost to threads in a space of an inch or more so that you've made a notch in the ring and then grab it and you should be able to pull up(not out) hard enough to get the then small amount of metal left to break.
If you're planning on replacing it anyway, there is little to loose! Just watch the threads when you cut.
As Archimedes said “Eureka!” I disassembled the headlight on my 2007 Big Dog K9 without breaking anything. I dislocated my shoulder patting myself on the back (both shoulders need replacing anyway).
1. I used liquid wrench to loosen it up. DON’T WASTE YOUR SPRAY. As old as that headlight is, once I removed the retainer ring(s) there was absolutely NO corrosion bonding the rings to the helmet threads. So liquid wrench is useless.
2. using a VERY SMALL flathead screwdriver and a rubber mallet I tapped the screwdriver between the “beauty” ring and the helmet. Make sure you line up the flat part of the screwdriver with the crack between the ring and helmet (that’s for my fellow Marines). I spent a few minutes tapping into the crack at small intervals around the ring. Then I went around the crack again (you know, like foreplay) and tried prying the ring up to loosen it. It didn’t cuz the little screwdriver would just bend. So I did my Zen thing looking at the headlight like a hog staring at a watch until I reached Nirvana and the clouds opened. I used three other larger screwdrivers putting one in the gap beside the little one. That loosened the little one and I took it out and tapped it about two inches away and stuck another screwdriver in that gap. I put the third screwdriver into the gap opposite the first screwdriver. Maybe some combination of 9, 6, and 3 or such. With the larger screwdrivers I took turns GENTLY prying the RING up and away from the threads (thank you (Forum Guy ____?). I was listening for the crack of metal that would have killed the patient but, despite the outside of the helmet bulging a bit the beauty ring cleared the threads and came out in one piece with minimal scarring (but chicks dig scars anyway, right?).
3. The retainer ring with three recessed holes behind the beauty ring I tapped loose with the small screwdriver in the recessed holes in a counterclockwise direction. I tried pulling it out when it was loose inserting the needle tip of a small tool that looks like a broken dick behind the retainer ring BUT THAT has to unscrew out from the threads or it might bend or break.
4. I did use the pointy end of the broken dick tool to pry the headlight out. Once the headlight was out I had to massage the rubber cover off the back of the headlight wiggling it back and forth. It was pretty snug.
5. used needle nosed pliers to UNHINGE (not remove) the retaining ring and pulled the bulb out the back end.
6. I started to stick a blue paper towel into the headlamp rear hole (Freud would be so proud) after spraying brake cleaner inside - (used a long metallic grabber to get it back out - cuz I almost bent or broke of the suspended reflector inside the headlamp.
7. The glass and the shiny reflecting surface inside the lamp were still dirty so...back to Zen...and I used a box cutter to slice the seam between the glass and the back of the headlamp. It only took an inch or two and the glass came right off. Cleaned the glass and the reflecting surface with windex. Don’t rub too hard or you’ll rub the reflecting surface off the mirror to see the brown material below it (kind of like primer I guess).
8. Big Dog HAS to find a better way to construct a headlight. It’s ridiculous having to buy an expensive headlight because no one can remove one ring to change a bulb.
9. Hope this helps You’ve all helped me a heck of a lot so maybe this will save you a few shekels.
Good luck.