So when you hit the change payment button, you did not select the Debit option? I think you are doing something wrong because yes by default MCGCs defaults to credit but if you hit the change payment button and select Debit it should prompt you to enter PIN as normal. You do have to hit the alternative pretty quickly after you swipe your card.

Now another problem that might have occurred is as soon as you slid your card the cashier might have hit the process button on their end too quickly which makes it more difficult for you to hit the change payment button. You have to make sure you hit the button before they do. To be honest its always best when they are distracted when doing this transaction because its always a race between you and the cashier on which person hits the process button first so you have to be on your toes. Perhaps this might have been the reason it wasn't working for you.