Can Porcelain Lumineers Be Placed over Existing Porcelain Crowns?

You can do Lumineers or other non prep veneers over existing porcelain crowns.  However, in most cases it will make the existing crowns look bigger and possibly bulky.  Bonding porcelain to porcelain is not a problem and you can get excellent bonding.  Yes, specific protocol from microetching the porcelain  needs to be followed.

Your question about the shade  chart:  Most veneers are influenced by the underlying shade of the prepped teeth or prepped crown.  Experienced cosmetic dentist and dental labs account for this and we design the veneers to with this in mind. What this means is that just because the veneer is made to a certain shade does not mean that the shade will be exactly the same unlesss the issue above is addressed. 

I hope this helps and was just curious if you have spoken to your dentist about the shade and why you are not happy?

Yes, veneers and Lumineers can be placed over existing porcelain crowns.  See and visit a Lumineer certified dentist to discuss all your concerns and questions.  It will not look "bulky" if done by the right dentist.

While this is theoretically possible, I would not recommend it. It would add more bulk to the tooth and you'll bet a better end result by redoing the crown.

Lumineers and non-prep restorations can be bonded to crowns. However, I do not recommend this method because it will add bulk to your teeth and make your teeth appear to be a lot bigger than they are. The best option is to remove the crowns and have Cosemetic Emax crowns placed. Good luck!

No point placing veneers over crowns, just replace the old crowns with Emax porcelain crowns and enjoy another 15 years problem free dentistry.

Actually, I had one of my patient's ask me the exact same question this week.  This isn't something I've done before, which is exactly what I told my patient.  It's also more expensive to place Lumineers than to just replace existing crowns in my office.  Even though lumineers can be as 0.3mm thin, the final result will still look a bit more bulkier which can be unesthetic.  Also we have no idea how well Lumineers will bond to porcelain, this isn't a procedure that the company advocates.  So why perform a procedure that's more expensive and also not very predictable?  I would recommend to just to replace the existing crowns if the shade really bothers you.  Good Luck!