If you're going to pre-prepare a story for the exam, you can't expect to manipulate any story perfectly (unless you get very lucky). Obviously, the better your skill in English / more creative you are, the easier you will find it to manipulate. Otherwise, prepare a few stories OR prepare only one but have a few ideas in your head. I'm stuck in the same dilemma with my half-yearly, and because I am amazingly uncreative I'm going in with a few stories though out already.