Never change textbooks. Changing books hardly ever significantly improves your class, so pre-tenure, it just doesn't make sense to invest that level of time in something that won't pay off.
That said, last year I switched textbooks in Intro to Biological Anthropology. I had inherited the previous textbook, and I had never liked it. It was full of facts, but poor on interpretation. The book did little to help frame the class within an evolutionary paradigm, or to explain why all of the facts they threw at the students were relevant.
Despite these drawbacks, I wouldn't have changed textbooks (after all, I provide the framework in my lectures), if I hadn't wanted to save my students a lot of money. The new book came in a $35 e-text version. All of the other textbooks are at least $100.
So, last year I put the work into converting my class to the next text. This year, when it came time to order my textbook, I found that the publisher had a new edition out, and this edition does not (yet?) have an e-version. The price tag for the new edition? $130!
I don't know if this is a deliberate bait-and-switch on the publisher's part. Perhaps they will create an e-version for this textbook next year, to which my students will have access until they update again. (And gee, nothing ever happens in Bio Anth to make them want to update, right?!) Perhaps it does take more time to get an e-version on-line than to get the physical book to press, but I doubt it. The e-version was little more than PDFs of the physical book, and surely the publisher has those!
I don't have time to make any changes for next semester, especially since I'm teaching a new prep in the Spring, and re-doing the third class significantly, but after this, I'm switching to Open Source resources.
Does anyone have any suggestions where I can find good Open Source readings for Intro to Bio Anth?