If a project has already been created with ARC disabled, how do I enable it and vice versa?
When you migrate a project to use ARC, the -fobjc-arc compiler flag is set as the default for all Objective-C source files. You can disable ARC for a specific class using the -fno-objc-arc compiler flag for that class. In Xcode, in the target Build Phases tab, open the Compile Sources group to reveal the source file list. Double-click the file for which you want to set the flag, enter -fno-objc-arc in the pop-up panel, then click Done.
Following are the steps which I did and it worked for me
ProjectTargetsBuild SettingsApple LLVM compiler 3.0 - Language and Objective-C Automatic Reference Counting and select NO in all three sections.
Open your project and select Edit -> Refactor -> Convert to Objective-C ARC. This will start checking your code if it is ready for the conversion.
Good link to read: Everything you need to know about automatic reference counting (ARC)
How to disable ARC has been answered here
In Xcode 5.0.2, select your project in Navigatior, select Build Settings, search for Apple LLVM 5.0 - Language - Objective C and change Objective-C Automatic Refence Counting to NO