No. Muscle and fat are two completely different tissues. Muscle could never turn into fat and fat could never turn into muscle. What happens is that when you stop weight lifting 2 things occur: your metabolism decreases, because your body now needs less energy and you also removing the body’s need to maintain muscle. The motto “use it or lose it” is also true for muscles. This way your muscles start to shrink and your adipose tissue increases, which gives the impression that muscle has turned into fat.