Do you want to scale the file so it will fit on a letter-size sheet, or adjust the layout to fit the sheet?
I need to scale the file. I know how to adjust the layout to fit the new US Letter. But after I do that, some of my content is past the bleed lines. Selecting all of the content and trying to fit it into the new layout obviously doesn't work without distorting the whole thing.
I'm assuming that I need to scale all my content into the new size, right?
Scaling means keeping the same aspect ratio for the page as a whole, but making it smaller so it will fit onto the lettersize sheet (and having wider margins), so everything on the page will be somewaht smaller. You can do this easily by placing the current pages into a new lettersize file. Scott Zanelli'a script makes the operation pretty easy in terms of positioning and scaling, and you'll find versions for CS3 through CS5 (which also works in CS5.5 and CS6) at InDesignSecrets » Blog Archive » Zanelli Releases MultiPageImporter for Importing both PDF and INDD Files
If you want to keep things the same size, which I think is a better plan for a folded brochure, but change the shapes and relationships between objects, you should use Layout Adjustment. Layout Adjustment must be enabled before you use File > Document Setup to change the page size. Up to CS5.5 you'll find it under Layout > Layout Adjustment... In CS6 it is hidden in the flyout menu of the Liquid Layout panel (also under the Layout menu). Objects snapped to guides will be resized when you change the page shape, but type size will be maintained. Images may be scaled or even distorted slightly depending on whether they are sanpped to guides as well.
In my experience, layout Adjustment is a bit unpredictable, and you'll have to do a bit of cleanup after, but it will get you started.