Although I'm perfectly fine with a 10 year old mouse, (at least it's optical) I had been wanting to spice it up, somehow. After a bit of thought I decided to fiddle around with the bland, overused red LED, and decided to replace it with a green LED, for no other reason than just to do so.
After some trial and error, I finally found the right size LED. I did make a slight error in lifting one of the traces that sourced power to the LED, but I quickly bridged it without any further damage. The results are pretty much as I expected; an ancient Logitech mouse that now shines green instead of red.
Was it worth the time and effort? Absolutely! Other than tracking down the right sized LED, the whole project took less than five minutes. It's super simple, besides me being hasty and messing up the trace, but it really is super simple. All I really wanted to do was change the LED color, because almost every stock optical mouse has a boring red light, but mine is different, which makes me feel slightly cooler than I used to. Only slightly though.