I just finished installing the SDK of WP8 and wanted to try my app now compatible with WP 7.1 and so I opened the project and compiled, but gives me errors only on the parts that use "async" and "await". I used Async CTP, but doesn't like much WP8. How can I fix and, above all, how can I make it so that the app functions both on WP and WP 8 7.1? Thanks a lot!!!
You cannot build Windows Phone 7.1 applications on Windows Phone 8 SDK without upgrading your project.
If you want to maintain WP7.1 version of your application with async/await feature, you will have to have Visual Studio Express 2010 for WP7.1 + Async CTP 3 along with VS2012 Express for Windows Phone.
UPDATE: There is a way to compile WP7 applications on Visual Studio 2012 Express for Windows Phone by using this nifty NuGet package. However, note that it is currently in prerelease version.