l have existing crowns and am wondering if they can be made whiter along with my other teeth.

NO BECAUSE THEY'RE ALREADY AS WHITE AS THEY WILL EVER BE,
If you bleach your teeth now, these crown will appear darker

Crowns usually stay the color they're made. l doubt that you can make them whiter. Ask your dentist, he will tell you for sure.

l have not done it but, l have seen Lumineers used over existing crowns (usually one or two). l usually tell my patients beforehand if they want their teeth whitened and the crowns no longer match and it bothers them, they need to have the crowns redone. It puts the whitening in perspective.

No it will not change the shade.

They cannot be made whiter with typical hydrogen peroxide. However, if you have had the crowns for a long time, it is possible the restorations have extrinsic stains on them. If this is the case, your dentist might be able to ''clean'' the extrinsic stains and make the crowns look whiter.

you can find reviews of teeth whitening products and procedures here: http://teethwhiteningreviews.com