How can I revert to normal margins while using marginnote s? In other words, in some pages of my document where I won't use marginnotes, I want the margins to be normal, as though I'm not using marginnotes.
It is not possible (or at least not really feasible) in TeX to change the text width at a page break, paragraphs are set in advance and broken into a series of lines, and then these lines are broken into pages. So you can not, after deciding the page break change the line width as the remainder of the paragraph has already been set.
If however you want manually break the pages with
\clearpage or similar commands then you know that the page break does not happen mid-paragraph so it is possible to change the width.
Easiest is probably to use the
geometry package which allows multiple page sizes to be set up.