Stuart and I are a little confused by your use of your terms in the context of Moodle, so here is some context to make sure we are "on the same page."
Courses hold activities and resources. The lesson module is an activity type. Each activity type works differently, especially in their potential interactions between student and teacher. Since this is the Lesson forum, I will outline some different ways to add and edit Lessons.
You can always add a new lesson or any Activities block to a course. For example, in demo.moodle.org in the "Moodle Features Demonstration" course, a teacher can turn on editing then add a Lesson Activity in any section. The teacher would probably hide the lesson as they built it.
You can always edit an existing lesson activity. For example, in the Moodle Features Demonstration course, a teacher can go to section 7 and open "Lesson 1 - Basic parts" and click on the edit tab. Now the teacher can select an existing page and edit the contents or any of that page's features. The teacher could also move pages, delete pages and of course add new pages like a branch table. Of course you can Import questionsinto a lesson either at the start or by inserting an import file between existing lesson pages.
Before editing, it is a good idea to understand the impact it is going to have on the students who are already in the course, or in the existing Lesson. Grading in the lesson (or course Grades for that matter) and any lesson dependencies and linking would be on my issues list to talk about with the teacher and how changes to an ongoing course would effect either new, current or former students in the course.
If the course is closed, say it is between academic periods, then the updating/editing task is easier. But you would still be wise to consider the impacts. Each organization will have different standards and while one will allow a teacher to create a new course and import all the content from the old, others will not.
TIP: In building new courses with many lesson activities, I sometimes use a Course backup and Course restore trick by using a Moodle site on my computer (I call these localhosts). This trick will also work on an existing course. Essentially, create a new course on the localhost and build the lessons in the sections you want them to appear in the live course. Backup the course, download the backup and then in the live course, upload that backup and restore it using the "existing course, add data to it". Sometimes working with pieces of a course like this is easier.
Be aware in any backup and restore process about any students enrolled in the course or on the site. Also dependency settings and internal links are best set on the live/real course, not the localhost.
Hope that helps. Chris