If the course is in a LP that is already assigned to users who may have completed it, the new course will not change their status. It also depends on whether the new course is something they would need to complete. If not, you can also create an equivalency between the old and new courses so those that have competed the old course automatically receive credit for completing the new course.

Alternatively if you prefer to only assign new users the updated plan, you can create a copy add your new course and then target only those users who are either in progress or have not yet started the LP.

If it is just updating the material, I add it as Training Material to the existing course and HIDING the outdated training material. That way it keeps completions for those who already did the course but hides the old version and only has the new version available. 

You don’t want to delete the old training material, as that takes away completions.