Apologies for the perhaps obvious question, but I'm still new to this. How does the $75 change work on a fare like this? If I were to buy this fare, pick my dates, and later have to change one of them, would I pay only $75, or $75 plus the difference between the fare I paid and whatever the fare has risen to at the moment I need to make the change? Thanks in advance for any clarification you can give.