So your site has been around for a few years and it's starting to feel long in the tooth. Maybe there are complaints about the way certain functionality works, or maybe there's a brand new version of the underlying software which is a drastic rewrite. However it comes about, in the course of any site's lifetime, you begin to itch for a change.
Certainly keeping a site's design attractive and fresh is important, as is regular maintenance and constant improvement. But when it comes to a full rewrite of the site, internet history is rampant with examples of sites completely redoing their interface and as a result frustrating and alienating large chunks of their user base.
Digg is a famous example of this. For years Digg had gotten regular maintenance and styling updates, but Digg v4 marked a complete rewrite of the site and drastic change to not only the look but the entire functionality of the site. Users left by the millions. Google has recently been redesigning many of their popular sites, not only updating the look, but in many cases altering the functionality and they have been hearing complaints from users, but so far there hasn't been any evidence of mass exodus from Gmail or Youtube. Twittter also redesigned not only the site but their apps on every platform; again sparking a round of complaints from users due to not so much the look but the functionality. Not only do users prefer the old functionality to the new one, but even in cases of users who are ambivalent about the changes, they are often put off at having to learn how to use the site again with no apparent benefit.
What's the common theme among these sites? They are sites that users visit regularly. They are tools for users to accomplish a certain task. They are destinations for certain activities. Knowing that fact, it becomes easy to understand why a drastic redesign is frustrating for users. If you went into your kitchen one morning and your coffee machine was replaced with a completely different one that was no better than your old one, and just worked differently, you would probably be looking for someone to throw your mug at.
So the question you should ask when deciding how far to take your redesign is, what kind of role does my site fulfill? Is it a tool or a destination like Twitter, Digg or Youtube? Or is it more of a sales or brochure-ware site, where users come to buy something and then rarely come back to it? A site where users do not make frequent visits and don't need to be intimately familiar with how it works. A site which is dead simple to use to begin with, where the only functionality is users placing orders for your products. In that case a major redesign is not disruptive to your users.
