Can u lengthen the veneers after being replaced permanently ?

No, your dentist cannot make your veneers longer after they have already been bonded into place.Did you have this discussion of the length of the veneers at any time with your cosmetic dentist before they were placed? Did you get to see them and approve of them before he permanently bonded them on? There is a great cosmetic dentistry technique that would greatly lessen the chance that you would be unhappy with the length of your new veneers. Your cosmetic dentist can use your temporary veneers as his template for your new veneers. This is called "the trial smile" technique. Too long to explain here, but photos and examples can be seen at the link below:

Generally speaking it is uncommon to modify the shape or length of a porcelain veneer once it has been placed. The reasons for this is as follows:1) Once the porcelain veneer is fabricated there is a final glaze that is baked over the restoration giving it a luster that protects against staining. Adjusting the veneer will remove this glaze, which can lead to staining and roughness of the porcelain.2) Bonding new porcelain to an existing veneer will create a line, although this can be masked initially with special techniques, the margin between the new and existing porcelain will discolor over time. To avoid future disappointments with the outcome of your porcelain veneers, I suggest you request a diagnostic wax up prior to preparation of your teeth. This will serve as a great tool for your cosmetic dentist to be able to show you the possible outcome of your veneers before performing the treatment. Hence you will be able to discuss your concerns before its too late.