If you want the existing content to scale, place the pages from the current file into a new file (any edits will need to be done inthe original, then update the links). The fastest method for doing this is probably with the script at InDesignSecrets » Blog Archive » Zanelli Releases MultiPageImporter for Importing both PDF and INDD Files
1. Make a square at the border.
2. select all (Cmd+A) and make a Group (Cmd+g)
3. Make an horizontal guide at 210 mm
4. with the selection tool (v) press Cmd+Alt+Shift and resize to the guide
Everything should be proporcional and you shouldn't miss text.
Check it out and let me know.