In order to run WPML All Import, you need to have our WPML All Import plugin installed as well as WP All Import plugin. When you report issues or ask for help, make sure to tell us the versions of everything you are running. This should include:
Problem: Is it possible to change the language of existing content?
Solution: I think the best way to do this would to be to bulk duplicate your pages/posts into the correct language. You can do this at WPML -> Translation Management. Then, once they are created, go to WPML -> Languages and change the default language to the correct language. Then you can delete the pages/posts in the language that you do not need.