I'm at home where I have an older copy of InDesign (CS3), and this isn't something that I need to do, so there will be aspects that I won't go into. Here's a sample document that I created with one page of text.
If I go to File>Document Setup and change the document size (I just changed it from portrait to landscape), it does this:
…where the document changes, but the content stays the same. If you go to Layout>Layout Adjustment and check the Enable Layout Adjustment checkbox (shown here):
… and then change the document size, you will get this:
As you can see, the text frame that was snapped to the margin guides has adjusted to the new document size, but there is still work to do because the amount of text that fit before no longer fits and has gone into overset. I just wanted to point that out because you will need to tweak your adjusted document to a greater or lesser degree, based on how complicated it is, so give it a try on a copy of your document, and if it totally screws up, you can go back to the original.
If the aspect and ration change, you have to take care that every frame aligns with the border guides and text frames for page numbers and running headers align outside to the margins, they have to snap to these guides. Distances to the text have to be done via text frame options. If the text size has to be changed it is to be done via changing paragraph styles. BTW you should always work with styles—Pargraph, Character, Object, Table and Cell Style.
If you need to scale proportionally, create a new document and place the other one either as INDD file directly or as PDF/X-4 into the document with the correct size. Interactivity is lost by this method.
Thank you so much for your help!! Since I'm working in ID CC I had to use the Liquid Layout feature instead of Layout Adjustment and change everything to "Scale", but this made everything so much easier!!