Make a copy of the file for backup. Resizng can go south very quickly.
If you need to maintain the same size type and want the document to reflow (and change the page count), enable Layout Adjustment (Layout > Layout Layout Adjustment...), then go to File Document Setup and change the size, then back to Layout > Margins and Columns, to adjust those. Things that are snapped to guides should resize to the new guide positions, those that are not will not change.
If you can live with placing your pages into a new file and scaling everything, use this script: InDesignSecrets » Blog Archive » Zanelli Releases MultiPageImporter for Importing both PDF and INDD Files
Any editing would then need to be done in the original version and the links updated.
So basically you have to do it manually?
Thats a pain, going to take me a while unfortumately.
The first method changes the entire document in two steps (after enabling Layout Adjustment, which must be done BEFORE making changes). The script automates placing, and scaling, all of the pages from one file into a new file. How much more automation do you expect?
Changing layout size is a pain inthe neck, and can, indeed be quite labor intensive if things are not set up to accomodate Layout Adjustment. Anyone requesting a change like this late in a design project should expect to have to pay for it.