Also, if you get a cassette tool now, you can remove the cassette and clean it up. Nothing like a shiny cassette!
Which brings up another point, if the old cassette has many miles, and you are installing a new chain, you might want to put on a new cassette to match the chain. If the old chain was "stretched", the old cassette wore to the elongated chain. Thus when you put a new chain on, it won't mesh properly with the old cassette.