Yes, you can make changes to your courses even after they have been published.
If you're looking to add new lessons, you can set them as draft first. By doing this, you can take your time to work on the lessons and it won't be visible to students already enrolled in the course. For more information on the steps to Create Draft Content for your Course, check out this article!
Once you do publish the draft lessons, they will then be viewable by students enrolled in the course! It is also recommended to then inform students once new content is ready and available for them to see!