How can I change the page format across all chapters?

[Celestra Erit368662070ocq]

It's quite a pity that synchronising the chapter files in a book does not include the page formats. I have a couple of cases here in which the books have to be reduced from A4 to US-Letter format without the texts and images changing their sizes. But when modifying one chapter to US-Letter and synchronising the book, the other chapters will invariably stay A4. Asigning master pages does not help, either, instead InDesign would complain that the master page has a different format than the page to which it assigned. It seems that the page formats (length and width, edges etc.) have to be manually altered for every single chapter, which is quite a time-consuming process. 
Is there any easier way how to automatize this rather awkward process?

[Eugene Tyson]

If you get a warning then the pages were resized outside of the Parent Pages - I believe. 

Hard to know from the description - but looks like pages might be different sizes to the Parent Pages

Reset your Parent Document to the correct size - and apply to all pages in your Parent Document

Then try to update the remaining documents. 

Perhaps on a test copy of the book file - you could package the entire book to another folder to test out a solution.