Convert derived class to base class

You can't - that's entirely deliberate, as that's what polymorphism is all about. Suppose you have a derived class which enforces certain preconditions on the arguments you pass to an overridden method, in order to maintain integrity... you don't want to be able to bypass that validation and corrupt its internal integrity.

Within the class itself you can non-virtually call base.AnyMethod() (whether that's the method you're overriding or not) but that's okay because that's the class itself deciding to potentially allow its integrity to be violated - presumably it knows what it's doing.

Although this sounds irrational but it works

 DerivedClass B = new DerivedClass();

BaseClass bc = JsonConvert.DeserializeObject<BaseClass>(JsonConvert.SerializeObject(B));