Cast base instance to derived class (downcast) in C#

Suppose I have two classes:

class Employee 
 

and

class AdvancedEmployee:Employee
 

I know something like this won't work, as I can't downcast on C#:

var employee = new Employee();
 var advanced = employee as AdvancedEmployee;
 

My question is: How to accomplish downcast in a efficient way?
Actually I have a constructor on AdvancedEmployee that takes a Employee as parameter and use it to inject its values, basically making a clone.

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