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.