I love to read stories about how small businesses convert to Linux from Windows. There are many paths to follow when converting Linux but there is always one common thread. The first is to take it slow and have the patience to work with individual users to make sure that they become more comfortable with working with the new operating system. In addition, if it is a business that make heavy use of applications, you should always try to find the open source equivalent of those applications to run within Windows first. This will make users comfortable with the open source applications before the big switch.
Here is a link to an interesting article about the conversion of Windows to Linux. Hey, and they are using Ubuntu too!