If you are considering making a big change to an existing site, the choices are always:

  1. Improve on the original and keep it generally the same
  2. Start fresh

Jakob Neilsen, the usability God, has some really good suggestions on how to decide between the two in a new article called “Fresh vs. Familiar:
How Aggressively to Redesign

Summary in 30 Seconds:

You may want to change the look of your site because you are bored with it. You’ve stared at it for hundreds of hours.

Your most devout readers/users aren’t tired of your site because chances are they’ve spent less than 30 minutes on it, ever. So don’t change out of boredom. Make incremental changes.

After a while of making incremental changes, your site will eventually be an awful hudge-podge of changes. Then, and only then, do you make a fresh start. But try to make the fresh start as familiar to the old users as possible while working all of the changes into something cohesive.

Hope that helps!