What happens if you cut down on fat and carbs and have a lot of sugar instead?

Sugar is pure carbohydrate so it makes no difference if you cut down on your other carbohydrates if the sugar intake is great enough. Furthermore a rapid increase in blood glucose levels will lead to the sugar being taken up by muscle and liver and converted to fat (in the liver) or a precursor like lactic acid (muscle) that is then converted to fat later on (liver).