Hi there, if you choose to replace, then the WordPress software assumes you are going to create the entire stylesheet yourself and totally replace the existing. What is better, unless you are going to do extensive CSS modifications, is to leave it as "add-on" and then you create CSS rules that will load after the original CSS and override the existing rules. One added benefit of doing it this way is that it is much easier to keep track of what you have and have not changed.
There is an add-on for Firefox called Firebug, which allows you to identify the CSS and HTML elements of the site and it will show you the CSS for that element. It also allows you to test changes in real-time directly from the browser which is a great thing. If you install Firebug and then take a little time to get familiar with it, you will find it an invaluable tool when doing CSS.
The following link will give you a little more information about using CSS here at WordPress.com: http://en.support.wordpress.com/editing-css/
I hope that helps. Please let us know if you have any questions or experience any problems.
If you would like to have the original stylesheet for reference, you can access it here and then copy it and paste it into a plain text file. Do not paste the entire stylesheet into the editor as it can cause image used in the theme design to disappear since the CSS uses relative links for the theme images.
I hope that helps. Please let us know if you have any further questions or encounter any problems.