As it seems there is no out-of-the-box way in Sencha ExtJS to provide a configuration based on the build environment (development, testing or production) to your application. Since you need at least different urls for your proxys, it makes sense to have a mechanism in place that would generate the respective configuration for you. This is our solution for the problem.
Currently we have to refactor some parts of an ExtJS4 application for one of our clients. During the refactoring process we renamed some namespaces to fit the current naming schemes, which changed a little bit over the time.