I have written an extensive application using Flex Builder 3. I have maintained the app for several years using FB3. In the mean time I have created several mobile apps using Flash Builder 4.7. So I am very familiar with the the new framework. However, my client has requested some big changes to the app. I figured this would be a great opportunity to migrate the app to FB4.7. I would like to keep the mx structure for the time being. So, are there any good documents concerning this migration path? What SDK should I use. Should I start using the Apache Flex SDK?