When exporting your project to a .folio file (used by Adobe’s Digital Publishing Suite for generating an iPad app), both the horizontal and vertical layouts are automatically included.
When InDesign converts a layout to new dimensions, it resizes and repositions objects based on Liquid Layout rules you defined or chose for each page or object. If the shape of your new layout isn’t much different from the original, you can choose a simple Scale or Re-center rule.
You can also drag out special Liquid Layout guides to control how different columns or rows of frames resize, or you can apply specific rules to individual frames to control whether they resize, from what origin point, and whether their content should scale. No matter how carefully you apply the rules, you’ll probably still need to adjust objects manually after the conversion.
InDesign can show you a live preview of how your objects will change. Just use the improved Page tool to drag the corner of your page to your new size and watch as objects move and resize.
As you might imagine, keeping track of changes that occur simultaneously in multiple layouts can be a logistical nightmare. Adobe’s approach to managing text and graphics that should remain consistent across multiple layouts is similar to the concept of placed graphics. When you place a graphic into an InDesign layout, it isn’t copied into the layout—InDesign merely remembers its location on your network. If you change the graphic in another program, InDesign’s Links panel warns you that it has changed and you can choose to update it in your layout.