Someone in the group knows how to change the main template of the course without having to retake the entire course again

The template dosen't quite work like that unfortunately as some parts are written into the main xml file as the pages are created so changing the templates and associated files won't change these.
If you were to change the support files, the icon images etc, and put them into the existing folders using the same names then it might work but debugging any fails would be a real pain.