OldVlaue get update everytime New vlaue changes

hello,
class Person
{
public string NewValue as string{get;set}
public string OldValue as string{get;set}
}
class  P
{
Person p1=new Person()                                         
p1.OldVlaue=P1.NewValue;
p1.NewValue="This is New Vlaue"      // in this scenario my old value is also get updated, i want assign oldvalue only first time
}
is there any was i keep the old value , whenever NewValues changes it also updaes the Oldvalue, I want only oldvalue get update first time from new value, later if New value changes old value must not update?
How can i do this?
Thanks