The best time to work out is the time that consistently fits into your schedule and enhances your day instead of disrupting it. There is no physiological benefit to working out at a particular time of day, so choose a time that you know you can stick with.
One tip if you do switch to morning workouts, though, is to eat a small, carbohydrate-rich snack as soon as you wake up. Overnight, your body uses up almost all the carbohydrate stored in your liver, and this is your brain's primary source of blood sugar. A PowerBar and glass of orange juice, or a bagel and a piece of fruit will give you the carbohydrate you need to get through a high-quality morning workout. Afterwards, eat a normal breakfast, with plenty of whole grain carbohydrates, to replenish your energy and power the rest of your day.