I've posted a reply to you on my blog (cause it's too long for here): http://bjclark.me/2009/07/03/how-do-you-redesign-something/

Excerpt: The biggest thing to manage in a redesign is risk. Risk you alienate your user base or do something that negatively impacts revenue, or whatever you're most afraid of.

The easiest way to manage it is to shorten your feedback cycle (not user feedback, any type of feedback) to the smallest possible interval. When talking about design, this means you need to spend the least amount of time on the design you can before you put it in front of someone that exemplifies your average user (persona).