If you change a nitride to an oxynitride film, of course the optical properties will change along, but of course that changes everything else as well (resistivity, magnetic properties, crystal structure...).

Adding oxygen via the gas phase is straightforward as long as you don't have things like filaments or other components which could burn, getting rid of the oxide residuals in your chamber after deciding that it's not what you actually needed is the much harder part here.

Jürgen Weippert thanks for your answer. I have found some technical information in a manuscript by H.A. Jehn in Chapter Decorative Coatings