On the Web: The 4 Catastrophic Mistakes of Any 
E-Commerce Website Upgrade

Along with big rewards, e-commerce website redesigns and major upgrades carry big risks. They require a high capital expenditure, lots of resources, and divert attention from the day-to-day business. Get it wrong and the ramifications can be huge. You've heard the horror stories of redesigns that lead to less traffic, reduced conversion and depressed sales.