I have been running the preview of Windows 8.1 on my Surface RT machine as well as my primary desktop at home for a few weeks now. There are some interesting and very nice changes that go well beyond just adding a “Start” button. One of the big questions is around applications that were developed for the Modern UI on Windows 8 and how they might be impacted by upgrading or migrating to Windows 8.1. Here is a list of resources for you that you should check out while you are playing with Preview of Windows 8.1 -
Windows 9.1 Product Guide for Developers - This is a High level summary of new features in Windows 8.1
Windows 8.1 New APIs and features for developers – Details about the new APIs for Windows 8.1
Windows 8.1 Application Migration Portal – Landing page for 8.0 to 8.1 application migration
Windows 8.1 Migration Guide whitepaper – Document on how re-target your apps for Windows 8.1
//Build Video – Upgrading and Migrating Windows 8 apps to Windows 8.1 - This talk walks you through the benefits of upgrading your app, shows you how to go about it in Visual Studio. Along the way, learn some helpful hints and tricks you won't want to miss.
//Build Video - Design and Build a Great Search Experience in Your App – Learn how to integrate the new search control into your app, best practices for doing so, and how to migrate your Windows 8 search experience to use the new control.
//Build Video - Beautiful Apps at Any Size on Any Screen - Design your app to be beautiful at all sizes, resolutions, and orientations! Scaling and layout topics for tablets of all sizes, laptops, desktops, high-resolution screens, different aspect ratios, portrait orientation, and multiple apps appearing side by side.
//Build Video - The Fast Lane for App Certification - Highlighting some of the patterns we’ve noticed in app submissions so far, and we’ll give you updated guidance and tips to help speed your app through the submission process.