I know you can change the deployment target when you build the app. Let's say I have version 1.0 of the app built with deployment target set to 6.0. If I now release version 1.1 and set deployment target to 7.0, the question is what happens to owners of iOS 6 devices running version 1.0 if they connect to itunes and sync? Will their 1.0 version be deleted or is it that they simply will not see version 1.1 available?
They will not be able to download and install version 1.1 unless they upgrade to iOS7. They will not lose version 1.0, that will remain on their device. Users with older iOS version (in your case 6.0) will be able to continue downloading version 1.0 even after 1.1 has been released.
See the article below for more details:
来源：https://stackoverflow.com/questions/19045995/ios-deployment-target-change-from-one-version-to-the-next