The theory is simple, it is a 90 degree rotated polarizing layer, How comfortable you are doing surgery on the delicate LCD assembly is another matter. If I were to do it I think I would buy a couple cheap watches to practice on first.
And Opinion: I wear a factory blackout casio, I like it because it does not have all that great 80's casio text all over the place (I am not sure what is wrong with me) but a downside is the inverted LCD display, it looks very cool, but is also much harder to read, I was trying to see if I could do the mod the other way, back to a light background.