A little backstory…
A few years back Fenix E05 made me fall in love with “wall of light” TIR optics. I’ve since put them in Convoy S2+ hosts and was very happy with the results (a bit of info and pictures in this thread).
I also love Baton series of lights. I enjoyed my S15 so much I got another, just in case 
But the second one had even more noticeable green tint, and after some deliberation (scared to destroy my not-so-budget light), I decided to try and put in a different LED. And I did, XM-L2 U2 1A on Noctigon worked, the tint changed to the rosy side, light survived, great success!
Fast forward to the present, I was trying to find a use for my SolarStorm SC01 (that horrible PWM in low mode kills it for me), thought maybe if I put it on my bicycle when I ride in daylight I’ll be less likely to see the actual beam and notice the PWM? But a light with a TIR, like my Convoy S2+ would be better in case it gets dark before I’m back… And bam, I got an idea: the head diameters of SC01 and S2+ look similar, would the same TIR fit? But wait, SC01’s head looks exactly like S15’s, and I already had it opened, so I decided to try that, instead. And it fits! The only thing I can’t stop thinking about is why in the world I couldn’t connect the dots earlier? 
It fits, but not quite. You have to do some work for the bezel to screw back tight. Forgot to mention I also have an S20, didn’t like the green tint (again) and smooth reflector, so I put a TIR in it, too. Here’s a picture with modded S15 on the left and S20 on the right:
As you can see, the TIR is a bit too long. The gap is bigger in S15 because of the thicker Noctigon board. Not a problem a little sandpaper can’t fix. For S15 I used P80 grit to get some material off fast, finished with P800 grit. S20 only needed P800. Here’s an original one compared to one sanded to fit the S15:
I didn’t measure, but I’d say it’s about 1-1.5mm difference, a lot less for S20.
The result:
Love that red Noctigon shining through 
If you want to try this on your S10, S15, S20, first get a TIR of your liking. I used this one from LEDDNA (they have 10, 25, 45 and 60 degree TIRs, also elliptical, frosted ones, whatever floats your boat).
Now you need to unscrew the bezel, remove the glass lens, o-ring and unscrew the reflector. When I did the emitter swap on S15, I needed to also remove the pill, and this guide helped me out. But if you’re only installing the TIR, I’d suggest to try and leave the pill in place, because that way you’re decreasing the chances of breaking the switch: removing the pill shaves off a piece of silicone button “leg”. I fixed it by sticking the cut piece in place with tape:
So if you don’t want to risk it, try it the way I did with the S20.
1. Unscrew the bezel. Method that worked for me is well described in the already mentioned guide:
I used a peace of old bicycle inner tube as my rubber pad. S15 was hard to crack open, S20 was a breeze, so your mileage may vary.
2. Remove the lens and o-ring. These lights use a custom o-ring which wraps around the glass lens, so they usually come out together. I simply hit the light on the table bezel down a few times until they reached the end and I could remove them 
3. Unscrew the reflector. The reflector is screwed into the pill which contains the driver. I lucked out it wasn’t screwed in very hard and just used my fingers to carefully unscrew it. Yes, there were some fingerprints left on the edges, but a simple cloth helped to remove them well, also I wasn’t planning on using the reflector ever again anyway.
4. Sand down and put in the TIR. As I mentioned, for tiny adjustments you’ll need to do P800 grit or similar super fine sandpaper works well.
5. Screw the bezel back on and you’re done. There’s no o-ring anymore, so waterproofness might take a hit, if that’s a concern for you.
That’s all, folks!
