If you used master pages for page numbers and headers/footers, changing that part should be easy enough. Text shouldn't pose too much of a problem either. But other elements of design might. Changing document size where the change isn't proportional is always hard, no matter which application is used, especially when enlarging it (for proportionally smaller document, one can create a PDF and use some of imposition tools to resize it; enlarging isn't recommended). In your case, I don't know of an automatic process to correct that, since you must enlarge the doc vertically, but shrink it horizontally. So, you'll most likely have to do things the hard way.
Some general tips:
1. As always, the first step is the most important: create a backup of your current document.
2. Repeating static objects like headers and footers (and page numbers) go to master pages. Depending on your project, you might need only three of them (first, left and right pages) which fit for all of the document pages, or maybe one set of two or three per chapter/section... etc.
3. Change document size in File > Document Setup > Document. Apply the new size to all pages. It is probably safer to go with new document, as John suggested, but I didn't encounter any problems with resizing the page yet.
4. Create guides around the page and enable snapping (Page > Snap to Guides), for easier repositioning of objects.
5. For those objects which are reused on different pages, but with different properties and/or position (so that they cannot be used on a master page), use Scrapbook (Windows > Scrapbook).
Good luck.