Upgrade - Importing customers and orders from old store

[Nils-H]

I'm upgrading my store from 1.4.3 to 1.5.4.1 and at the same time, I'm restructuring the product catalog. The new product catalog is already intalled, and now I want to import the customers and the orders/order history from the old store. What is the best/easiest way to accomplish this? I only want to import the customers and orders from the old store. Not the old products and categories.