Three viable solutions:
1. Don't do anything. You heard it right! Ask the printer to magnify/reduce your PDF to the correct size (you can try out a few percentages on a good photocopier).
2. Export your current doc to a PDF. Make a new ID document of the correct size. Import the entire PDF and place them at the correct size. Don't worry, no manual labour involved, there are Javascripts out there that make it a fairly painless process. Downside is if you alter something in your original text, you will have to re-create the PDF and update the image document.
3. Make a new ID document of the correct size. Import and place the original ID document into this (there are scripts for that as well). Works almost the same as (2) above, but in this case, editing is a snap. Just use “Edit original” on the image of an ID page and ID will open the original document into its own application — InDesign itself.
Am I missing something? I don't think so — adjusting layout is possible with the document you have right now, but you have to scale the text frames “manually”. If you just resize them, the text will run radically different; and ID is just too good with typesetting to assume “if I reduce the frame by 10% and I make the text 10% smaller, it'll be the same” — it won't.