Do you regain weight as soon as you stop training?

Someone told me years ago that if you stop training then very quickly you will regain the weight that you lost because your muscle will turn to fat. Is this correct?

 

Muscle tissue can never change into fat tissue because the composition of both is different.
Muscle tissue is made with amino acids while the fat tissue is composed of fatty acids.
Muscle tissue is around 700kcals and fat tissue is like 3500kcals per lb.
You would regain the weight if the weight you lost was water weight. If you lsot fat then it won't come back. although the body would try to bring back the body again to fat but if the diet is just somewhat controlled, it won't happen

 

So if you lost weight and then reduced your exercise regime, as long as you retained a healthy diet, the fat/weight loss would not necessarily return?

I find your attention to detail fascinating and very very interesting.