Jared Spool of User Interface Engineering published a fascinating article way back in 2003 about the death of the major site relaunch:
[T]he best sites have replaced this process of revolution with a new process of subtle evolution. Entire redesigns have quietly faded away with continuous improvements taking their place. The big survivors of the dot-com crash -- Amazon, eBay, Dell, Google, Yahoo, and CNN -- have each foregone the big redesign in lieu of continual changes on the site. The changes are so fluid that users hardly notice.
The article centers around what I would imagine is a pretty common scenario: a small company that has become totally dependent upon its dated, unusable, and ugly website. Jared’s recommendation is to follow the examples of the big web-based firms and slowly evolve your site design towards something you wouldn’t be embarassed to show your mom, instead of embracing radical (buggy, expensive, time-consuming) change. The same principle applies to software development. It’s rarely advisable to throw out your entire codebase and try to rewrite from scratch. After all, look at what happened to Netscape when they tried it.
Jared makes a note that many users don’t even realize that Amazon, eBay, Yahoo, and others have slowly morphed their pages over the past several years. If you go back and take a look at Amazon from the end of 2000, their site looks dated but very close in form to today’s appearance.