Sometimes a course gets an update that warrants some extra costs for your currently enrolled students. However, if you have evergreen students (no enrollment expiry date) you'll need to make a new course if you want currently enrolled evergreen students to purchase the new material. Kicking all of your students out of a course without warning or prior agreement to this condition of their purchase makes for really angry students! Avoid doing this by planning ahead.
If you don't have evergreen enrollments in your course, you can set up a system where students in a course have their enrollment expire on a certain date where you plan on updating the course. Then, if they want to re-purchase to gain access to the course for new material, you can time an email campaign to coincide with the course update and the end of their enrollment where you send out a discount coupon code that negates all (or some) of the cost of the original material from the updated course's price to encourage re-enrollment.
Before this update date, you can add material to the course as draft content, and then publish it on launch day. Or, you can set up a drip schedule where all the new content is released on the day of the update for everyone.
Many times, these are the situations that can have their complexity mitigated by a subscription price where a student pays a recurring fee to retain enrollment in your courses which are getting updated frequently.