I recently uploaded a test version of my app to iTunes Connect and the next day got this email:
We have discovered one or more issues with your recent delivery for "MyApp". Your delivery was successful, but you may wish to correct the following issues in your next delivery: Missing 64-bit support - Beginning on February 1, 2015 new iOS apps submitted to the App Store must include 64-bit support and be built with the iOS 8 SDK. Beginning June 1, 2015 app updates will also need to follow the same requirements. To enable 64-bit in your project, we recommend using the default Xcode build setting of “Standard architectures” to build a single binary with both 32-bit and 64-bit code. After you’ve corrected the issues, you can use Xcode or Application Loader to upload a new binary to iTunes Connect.
The last time I did a release I couldn't upload the app until I fixed the 64-bit build so I know that's okay, but I'm still building against the iOS 5 SDK because our app still supports it. If I change to build for the iOS 8 SDK will this stop my app working for older versions of iOS? How can I tell if new functionality will still work on iOS 5?