IME, only flights with the same fare class available show online or via app when requesting the change. I'm guessing you'd need to call if you want to change to a flight with only a higher fare class open.

To the OP: once you check in online, you can see a list of flights available to change to within 24 hours. So at 5:05 pm Thursday, you'll see a list of any available flights later Thursday evening until 5:05 on Friday. Since you're looking to switch to an earlier flight, this would work. But next time, if you are looking for a later one, you need to wait until 24 hours before that flight time. So if you wanted to get on the 9pm friday flight instead, you'd need to go back to the check in screen at 9pm Thursday and see if it was available. If you wait until noon on Friday, you'll see avialability up until noon Sat.