It could have been automatic to adjust the text frames had you done a bit of preparation, but now that you've done the margin adjustment the automation may not work.
There are two ways you can create threaded text frames, either by creating a frame on your Parent Page, and using that when placing text on the document pages, or by or by placing text on document pages that have no exiting frames at all, and the relationship between frames and margins is subtly different depending on the method.
If you are using a Primary Text Frame from the Parent page, adjusting the margins on the parent page and adjusting the primary frame there should propagate across all pages based on the parent, as long as you have not changed the dimensions or position of the frames in any way on the document pages, whcih would break that link.
Non-primary frames, as long as they are snapped to guides, can be adjusted automatically by simply checking the box to Adjust Layout in the bottom half of the Margins and Columns dialog under the Layout menu, but you have to do that BEFORE you click OK and leave the dialog. frames no longer snapped to guides won't adjust after the fact.
We know that the book was 347 pages, but not how many threaded stories or how it was set up. If the number of stories is small, it might be faster to reflow the text into a new document with the revised margins rather than try to adjust frames on hundreds of pages. You can select an entire threaded story by place the cursor anywhere inside it and choosing Edit > Select All, then copy to the clipboard, and paste into the first frame of the new document. Again, how this document is set up will affect whether the text flows to multiple pages, but if it doesn't auto-flow on its own, pick up the overset text by clicking the red + sign in the out port of the first frame, hover the loaded cursor over the next frame, hold down the SHIFT key and click to flow the text. If there are no frames on your pages, click the loaded cursor on the top margin to create a frame that fills the margins. Clicking below the top margin will make a frame that fills the column width and extends from where you clicked to the bottom margin. All things being equal, it's probably easiest to put text frames on the Parent Pages in the new doc befor moving the text.
If you have graphic content not anchored into the text that can be moved with Copy/Paste as well.
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