Can I Change for Veneers Having Already the Crowns?

While in theory a veneer COULD be placed over some crowns, the fee is the same as replacing the existing crowns.  Replacement would be best, especially since you are reporting pain.  It is possible that the nerve of one or more teeth is involved and should be addressed first, then new all porcelain crowns should be placed.  

When done right, all porcelain crowns CAN look fantastic and very natural.

Unfortunately, as you've already heard, you cannot replace a crown with a veneer. The lesson to be learned, however, is that not every dentist can do cosmetic dentistry. Also, remember: "You get what you pay for!". True cosmetic dentistry requires time, expertise, and the use of an excellent cosmetic laboratory, all of which cost precious dollars. If you do chose to have the crowns redone, shop around for quality, not price. Don't accept the final crowns until youy are very happy with the temporaries. Also insist on seeing the permanent crowns on your teeth with a "try-in paste" and make sure you LOVE them before allowing the final bonding or cementation.

Unfortunately once you have crowns you cannot go back to veneers, but All Porcelain Crowns can look as good as veneers if the are done well. Many of the cases that I do are combination cases with crowns next to veneers and you cannot tell which tooth was treated with a veneer or a crown.  What you have to realize is that the tooth structure underneath the crown or veneer will severely influence the final look of the crown or veneer.  If we are trying to make a significant color change for dark stained teeth the porcelain will need to be more opaque and not as transparent.  You may have a difficult case to treat.  Find a cosmetic dentist on AACD.com and get a second opinion.  Sorry for your trouble.