Eating low carb and switching to high carb will cause significant water weight increase due to increased muscle glycogen. On the order of 5-10lbs. It's not fat.
Excess calories can be stored as fat. Carbs that drive high insulin levels end up being converted to fat, and the subsequent low blood sugar level promotes more carb consumption.
This does not happen with protein and fat consumption. If you force yourself to overeat those foods in the absence of carbs, which is not easy, they will be simply eliminated.
You're saying that eating excess calories in the form of fat won't be stored as fat? That's the most efficient pathway and was critical to our survival.
What you're referring to (excess carbohydrates converted to fat) is denovo lipogenesis which is exceedingly inefficient and rare in humans.
Not the person you addressed, but people are not lab rats whose daily caloric amount is controlled by a lab technician.
We humans in rich countries eat ad libitum. Any dietary intervention that helps us feel less hungry will lead to a reduction in the calories we consume.
But if I continue to eat high carb, there is a weight gain that is much harder to shed.