It depends on your behavior. If you are simply replacing your old high fat habits with reduced fat substitutes but not really changing your overall caloric intake then you may be substituting one problem for another. Sugar is a simple carbohydrate. It is very easy for the body to utilize and convert into energy. Complex carbohydrates such as fruits, grains and vegetables require more work to break down and may contribute more to sustained energy levels than simple sugars.
Sugar adds nothing to the diet except carbohydrates.
Fruits, vegetables and grains can add necessary fats, vitamins, minerals, proteins and fiber to the diet in addition to carbohydrates. The impact of pure sugars on the diet is limited and so the calories introduced by these foods are often called empty calories. The more complete foods provide more value for the calories they contain. We gain weight because we consume more calories than our bodies use. The cornerstones of a good weight management program are diet and exercise.