Process Involved in Replacing Composite Veneers with Porcelain Veneers?

Make sure the dentist is experienced with this type of procedure. You should have pictures and study models taken to do a "wax-up" or mock-up of what the veneers will look like. You don't build a house without plans (blueprint), so you shouldn't build teeth without a wax-up (plan).  This plan is based on your particular smile, and tooth display desired.

Composite needs to be removed to get to clean tooth structure for proper bonding. Teeth are refined to establish margins. The amount of tooth reduction is dictated by the wax-up. Its possible that just by removing the composite there will be enough reduction. Generally need 0.5 to 1mm depending on situation. Impressions are made of prepared teeth and sent to lab. Temporary veneers are made by dentist before you leave the office (made from wax-up). The temporaries will protect the teeth from sensitivity as well as have you looking good for the 2 weeks or so that it takes to make the porcelain. Also, the temps are a "trial" set that you and the dentist can tweak to your liking. An impression of the temps taken after tweaking should be sent to the lab as guide for the finals.

Hope this helps

Dr. T

The composite veneers should be completely removed if they are to be replaced with porcelain veneers.  The teeth will be protected with provisional or 'temporary' veneers while the porcelain veneers are fabricated.  These temporaries will seal and protect your teeth during the interim, and they should prevent any sensitivity issues.