Of original list I would say Peak Led is likely quite hard to mod because of epoxy potting. Arc AAA may be easier but I don’t think I’d want to mod any of mine, out of tradition or something. Princeton Tec Aurora headlamp should probably get a resistor change. Actually maybe a lot of these lights should get that, due to newer leds having lower Vf(?). The Aurora runs pretty hot as it is, as does the 4aaa original Attitude. The Attitude is likely a good mod candidate though.
If my math is right, the Nichia GS led used in the later Arc AAA, Fenix E01 etc. was close to 100 lumens/watt, not that far off modern leds, though its tint was awful. But that means the earlier leds were a lot worse. It might be worth going back and modding some of the old Tek-tite lights that were pretty bulletproof and therefore still of some interest.
Lights such as Dorcy were moddable but maybe not worth the hassle in this day and age.
I got some good leds from MtnDave and will try modding a few coin cell ights first and going from there. I didn’t get any of rngwn’s this time around. They looked great, but shipping costs from AU would have swamped the cost of a small order.
I have not yet opened up my Photon Freedom and would be interested to hear how to get it open. More ambitiously than changing the led, I wonder what it would take to reflash or change out the microprocessor in it.
Sofirn C01 at least in some versions has a nice high CRI led in warm white already, so why mod? They are just $4 each and are very nice.
Pak-lite, similar to Arc: it’s cool because it’s ready made, but if you want to substitute the leds you might as well make your own version of the light. Two leds in series with a resistor, plus a battery clip that you can scrounge from a spent 9v battery.