How can I make sure that every single page's margins are changed?
The reason it wasn't working for you is that in the document every chapter has it's own (different) page style.
To fix this:
Press f11 to display "Styles and Formatting"
Select "Page Styles"
Delete all of the "Convert xx" pages styles (select the styles one by one and press del).
- All pages will now have the "Default Style" page format.
Select the whole document ctrl+a
Change the margins.
All pages will reflect the new margins (and retain the Page Layout "Mirrored" value).
Notes:
Every chapter has it's own (different) page style.
You can see this if you goto "Style and Formatting" (press f11) and look at "Page Styles" when you click through each page.
The "Page Style" changes from "Default Style" > "Convert 1" > ... > "Convert 18".