Removing coatings from lenses would involve exposing the lens elements to some pretty horrid chemicals, cleaning, re-polishing and then re-applying the coatings. It would be too expensive to even consider.
Originally posted by Zav I am not aware of Sigma recoating lenses.
Nor have I, though they do offer a mount replacement service.
Originally posted by Sandy Hancock Not to mention that re-coating may well change the properties of the elements enough to change the whole optical formula of the lens.
Coatings are uniform, microns thick and multi-layered. I have tested lenses with the same optical formula with different coatings and I have yet to see any lens that exhibited drastic changes in optical characteristics* with newer coatings. However, Image contrast and flare handling is almost** always improved with modern coatings.
* I have seen slight changes though I am hesitant to state this as an absolute fact, as I would need a greater sample size to tell if the differences were caused merely by sample variation.
** unfortunately some lens designs will flare badly no matter what you do, new coatings or not.