The length of the centrals usually should be at same Invisalign edge position as the canines. They should follow the curvature of your lower lip. In your case, they do seem a little short. If the centrals are lengthened, the laterals would also need to be lengthened. An easy exercise to see how the additional length woul look would be to have your dentist add a small amount of composite to each veneer to increase their length. This would only be placed temporarily, not chemically bonded to the veneer. This would give you an idea if the new length is really what you want and how it would look. If you do decide to change the veneers, I would only change the four front teeth. All that being said, your smile looks pretty good the way it is. The ultimate question is If you are happy with them. If you are not, then I would definitely have them removed and lengthened.