It depends. If it's a big dark, poorly done tattoo, it might be hard. If it's a small, light, faded tattoo, it will be a lot easier. Make sure you get someone really experienced with coverups. Look at samples of their work. I've seen coverups that were worse than the original.
I was getting tattooed one day and this woman comes in with one of the most amateurish, God awful tattoos I've ever seen. It was a cover up. After she was gone my artist looks at me and said, "That looks like Arnold Schwartzenager climbing out of a tar pit."