Given two choices (equal probability; redundant?) - which is the case once you have two doors - switching cannot change the odds. Have someone flip a coin. Call

Given two choices (equal probability; redundant?) - which is the case once you have two doors - switching cannot change the odds. Have someone flip a coin. Call it. But before the flipper looks at the coin - (whether he KNOWS the result or not) he asks you to consider switching. Explain how the option of switching changes the probability of a correct or incorrect call. Inference?