Porcelain Veneers Turning Yellow

One of the advantages of porcelain veneers is that they are not supposed to stain or turn yellow. But after 15 years Erik is experiencing yellowing veneers. He asks this question in a dentistry forum;

Hi

I had six porcelain veneers done about 15 years ago in the states and very recently I have noticed that two of my porcelain veneers are turning yellow around the edges. I asked my hygienist to remove the stains and she tried but could not remove most of these yellow stains on my veneers.

Could you tell me how I can have these stains removed? Is it the case that those veneers will have to be replaced or can the stains be removed without removal of the veneers?

If I have to have the veneers replaced, how quickly will the new ones be made? What will I have on my teeth whilst I wait for the new veneers to be made?

His question hasn't been answered yet. Could a cosmetic dentist please answer him here?