All right, thanks for the advice, everyone.
Just wondering, what would happen if I missed my flight and showed up at the airport for a later flight? Would they put me on standby for the next flight, or would I have to buy a brand new ticket at the walk-up rate?
I believe that depends on the airline and the fare class. For cheaper, more restricted economy class tickets, if you are late, you will probably wind up with a no-show or re-booking fee at the least. Since it sounds like the change fee on the ticket you have is rather large, that is probably the minimum of what they would charge you if you were to show up late to the airport. In the worst case scenario (the most heavily discounted tickets or ones from budget carriers), you forfeit the entire ticket and you must buy a new ticket at that day's price.
That's why if you can afford it and you are not 100% sure of your plans, it is best to book directly with the airline and pay for a higher fare class giving you more flexibility. It is always a good idea to check the fare rules for the airline.