Uploading test build To iTunes Connect before integrating GameCenter confusion

Before I upload my xcode project for testing purposes so far, do I have to integrate Game Center's Leaderboards and iAd first for my game app or can I upload my xcode build for sandbox testing and then integrate Game Center and iAd through that uploaded project? I want to test Game Center with Sandbox testing but don't know if I can upload my xcode project first and then put in Game Center and then do sandbox testing through that upload? Or Once I upload to testing, I can't change any code in xcode project?