Answer: No. Here’s what really happens...
Muscle cells cannot turn into fat cells. It’s like you can’t make a brick of gold out of a piece of lead. If you stop working out with weights/resistance training your muscles will start to decrease in size (atrophy) after a while. With a loss in muscle mass comes a decrease in metabolic rate: you will not be as efficient in burning calories and thus you are more likely to gain fat.
Fat cells will increase to store the extra calories you are no longer burning with your weight training and higher metabolism. So your overall body composition changes with a loss in muscle mass and a gain in body fat, but you are not substituting muscle cells for fat cells. So keep up with the resistance training regime: be sure you have an effective and safe program and you will maintain your healthy physique.
Always consult with a physician before starting an exercise program. Stop an exercise if you feel pain.