If you are considering making a big change to an existing site, the choices are always:
- Improve on the original and keep it generally the same
- 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!