I am trying to change the default langauge of a post that has already been created with a different default language, since we switched from english to danish as the default language, but many of the older posts still translate from english.
the sites default language is Danish.
Welcome to the WPML support forum. I will do my best to help you to resolve these issues.
Please try the following steps after a site backup and let me know your feedback.
1) Open the post for edit in Danish
2) Choose *none* for *This is the translation of* field of *Language* widget(on the right side of the screen)
3) Update the post
4) Then open the English post(original post of Danish) for edit
6) Then connect it to the English post(from the Language widget)
7) Set Danish as the original language
8) Visit the posts in the frontend and check you are getting the expected result or not
Please check the attached images for more details.