If you've setup your document with margins and guides etc. then scaling it should be ok if you go to Layout>Layout Adjustment and turn this on.
Then change the size of your document to the new size. You will have a bit of tidying up but it can work.
Your other option to get you out of this now are to import the InDesign file into a new document at the right size. - http://indesignsecrets.com/zanelli-releases-multipageimporter-for-importing-both-pdf-and-i ndd-files.php
Or you could request for your printers to resize it this once.
But you should of course resize this to the correct size.
Hmm, none of them really worked.. Any other solutions i could try?
Did you press the keys before you clicked the mouse?