When opening projects that were created in an earlier version of Xcode, you can update (modernize) project files to ensure projects are stored with the latest Xcode file formats and are in line with the most current features in Xcode.
To update a project (if Xcode does not notify you when opening a project):
– Switch to the Project Navigator
– From the Editor menu, choose Modernize Project
Interestingly, when viewing the Editor menu, sometimes an option to Validate Settings (versus Modernize Project) is shown:
In either case, a dialog box appears listing a number of options you can elect to have updated:
Once complete, your projects will be compatible with Xcode 4.
One additional option to initiate updating a project is to click the Modernize Project button (shown below) from with the Project Navigator when viewing the Build Settings – thanks Andrea for the email pointing this out: