I've written four non-fiction books for middle and high school students. Three were for Scholastic. In the first one, the editor left mid-project and the new editor wanted a breezy, less documented style of writing, so I had to remove a lot of stuff in the text that indicated where the data or information came from. Guess what? When the book got reviewed by School Library Journal, the one thing the reviewer criticized was that the lack of documentation - information I had specifically been told by the editor to remove.
Book two - nothing substantive was changed, just minor editing for clarity.
Book three - This one comes out next month and has been through two editors and a book packager. Books 1 and 2 were part of a series that won an award, so the publisher decided to re-design and re-name the series to give it higher visibility and aim it at an older age group. The re-design took a year. One editor left, then the project was given to a book packager, then another editor. Most of the information was left alone, some of the text and examples were moved into sidebars and other stuff was done to make it more visually appealing. I think it made the book more appealing and accessible and with luck, more saleable.
Book four - This was with a different publisher and it was a nightmare. The book was at the copy editing stage when a new editor took over. The book dealt with a topic that at that time was heavily covered in the daily press. The new editor did not like the way the topic had been approached in the book. She canceled the book and I got to keep the advance. I think I may have been caught in some office politics at the publisher's where the new editor wanted to make her mark, as she seemed to be excessively interested in discrediting the former editor. The book had gone through two edits (one fairly major, one minor) with two different editors and had not only been approved to move on to copy editing and fact checking, but had been praised by them. Both these editors had abruptly left the company shortly before the new editor took over. I got to keep the advance without a fight. All parties agreed I had met my contractual agreements, but the new editor hated the book and felt it did not fit into the publishers line. All I can say is that it is a good thing this wasn't my first book (It was actually my 7th, but some were ghosted, and I had no control over the finished product) or I would probably never have written another book.
So the short answer is that amount of changes depends a lot on the vision of the editor and the publisher of what the book should be, and there is no way to know until you start working with someone what they will want.
