On very very rare occasions, its possible to cover up an existing tattoo with something that may be relatively the same size. But most times, if not every time, a cover up is at least 3 times the size of the original.
I know, its hard to re-commit to something that's 3 times the size of the tattoo that you now regret, but it really is the best way to cover it up, without having to cover it up again. Trust me, Ive seen that happen.
Its always best to choose something that is detailed and that has large dark areas. In this case, Nelia decided to cover up some unwanted to script with a flower side piece. Although the flowers themselves may be light, the darker parts like the leaves and the heavier shaded colors will be placed over the old tattoo. Sometimes the idea isnt to place the cover up right over it, sometimes its better to distract the attention.
But dont just take my word for it. Wait until its all done. You'll see what I mean.