It is ALWAYS best to do veneers at the same time. Ceramists do an amazing job of matching porcelain to teeth; however, porcelain comes in dye lots much like porcelain tiles. Therefore, you want to have veneers made from the same dye lot. It's also advantageous for purposes of teeth alignment and optimizing teeth shape, size and texture. Redoing a veneer that is already done however is a choice you and your dentist will have to make together because that is a sizable investment for potentially very small rewards and must be evaluated on an individual basis. That being said, it is the front of your mouth and is seen and noticed by everyone you interact with.
Good luck!