Assuming that If you are not satisfied with shape and Colour of your existing crowns and you want esthetically better looking crowns,there are all ceramic crowns available which have excellent Esthetics and Marginal accuracy.
It can be done in single visit if your dentist uses chairside CAD-CAM unit provided you don't have any infection or pain in teeth.
It takes 2 visits to get it done.
Next Steps
Visit a prosthodontist who specialises in crowns, Bridges and Implants.
Health Tips
If you feel your existing crowns are moving or mobile, avoid eating hard food and immediately visit your dentist.