There is always going to be some manual work involved in reformatting a book like this, especially with the images, particularly those that are not anchored and may need to be repositioned. Layout Adjustment can help a lot -- I use it for this from time to time -- if you turn it on before making any changes, then make sure the margin guides are set to the size of the main text block before you attempt to make any change to document setup.
Once the margins are correctly setup you can use Document Setup to change the page size, and then go to each master page and change the margin settings to what they should be, if different, and to move master objects that you would not be overriding. That should get you 80-90% of the way to your new layout, and you can save the file with a new name. You want to avoid reapplying master pages that have objects that you've overridden.