Where is the tattoo going to be placed in relation to the cover up tattoo? In the case that you are placing the cover up tattoo directly over the other area where the tattoo had been placed, there are often changes which must be made through the texture of the tattoo. You are often restricted when it comes to changing, as the solid colors must often be used to cover the tattoo that had been placed under the tattoo.
Although you are limited to shading, working with a talented tattoo artist can ensure that you are able to create a tattoo that is not too much larger than the one which is being covered. In some cases, you are even able to create a tattoo that is the same size as the one being covered. Be sure to check the past work of the tattoo artist for examples that can help you to make your decision about who to work with.