I agree with Lightbringer but if this is as you say ‘for fun’ then there’s a few good suggestions above. Also, if the fun is in the modding then just do it but don’t expect to get much back on your investment ( there’s a reason so many people swap out leds themselves ).
Main points being
1. you won’t know exactly until you actually do a small run so do some research on what people might want.
2. acab is spot on about original cost of the light. Additional costs on an upper range light seem less because it’s less markup percentage wise. You don’t see Vinh (or others) modifying cheaparse 3*AAA lights. He modifies mid-range to expensive lights and only the really popular cheaper ones.
Otherwise have at it and I look forward to seeing what comes… 