In his article Good Designers Redesign, Great Designers Re-align Cameron Moll talked about not redesigning a site completely and certainly not just for aesthetically reasons but mroe a redesign (or realignment) should be done purely for the purpose of improving the goals of a website.
Really small, some might say insignificant design changes can have large effects on your conversion rates, for instance at work last week, at the request of a colleague, I moved the add to basket form from below the product description – which for most people was below the 1024*768 page-fold – and low and behold the percentage of people adding items to their basket increased dramatically.
A small change plus another small change and another and so on and so on add up to a big change but without the disadvantage of alienating your customers who have to get used to your old design. By slightly changing certain aspects of you site you can upgrade your customer’s experience hopefully without them noticing anything has changed.