I see the additions and deletions as covering aspects of revising, but you are correct about the scope and difficulty. Often, revising is the most difficult challenge when it comes to modifying an existing textbook. Being more of a technician, I tend to focus on the technical challenges, like revising content in a PDF document (virtually impossible without exploding the revision scope to include reformatting the entire book), or trying to figure out how to author in LaTeX when you may have never worked with it before.