Some people can develop an abnormal heart beat known as "Atrial fibrillation" or "AF" that puts them at risk of developing clots in the heart. Those clots can travel in the blood circulation to the brain and cause a brain attack ("a stroke"). To prevent those clots forming, blood thinners (anti-coagulants) are used. Apixaban is a blood thinner that works by stopping one of the blood substances required for clotting ("Factor Xa"). It is approved and used to prevent clots forming in people with "AF". Other established blood thinners work by stopping clotting substances being made, known as "Vitamin K antagonists" or "VKAs". An example of this type is Warfarin (Coumadin). The good effects of all blood thinners are preventing clots, and they may also have bad effects of increasing the chance of bleeding. People with "AF", abnormal heart beat, may benefit from changing it back to a normal regular rhythm, known medically as "cardioversion". When this is done, people are currently most commonly treated with a "VKA" blood thinner (e.g. warfarin). The purpose of this study is to assess the good and bad effects ("efficacy" and "safety") of apixaban compared with warfarin in people with "AF" in whom an early cardioversion is planned.