Although both function as leavening agents, it's not a good idea to substitute one for the other in most recipes, including this lemon crunch pie. This is because they have different properties and baking powder (and baking soda) add a flavor that's different from the more neutral taste of cornstarch. Bear in mind also that cornstarch is known as cornflour in some parts of the world (such as the UK), but it's the same thing.