No other airline in the US cancels and changes flights like Delta does. That being said, American, like the other legacy carriers, does reassess loads each quarter and MAY change the type of aircraft you fly, or adjust the time of the flight by 20-30 minutes either way. These happen no more than once a quarter (if at all) and generally the only time your flight(s) may be adjusted is the first week of the second month of the quarter previous to the one you're flying in. If you fly in April 2008, the time frame for any possible adjustment to your flights will be the second week of February. American will not cancel the morning non-stop, only adjust the flight departure and arrival times by a few minutes in either direction. You will have the opportunity to pick your seat when you book your tickets online. You can change the seats as often as you like, provided they are open. American flies B757s non-stop from LAX to MCO and flies MD-80s, B737s and B757s on flights that connect through DFW. The morning non-stop from LAX is a nice flight, I have taken it several times. I like that it gets you into MCO before 4 pm. Gives you time to make dinner reservations some place...
American is a convenient way to fly across the country because you can either go non-stop or connect through Dallas, and they have many flights throughout the day. I am not a big fan of red-eye flights, so I like the morning flights, even if it means taking an extra day of vacation
I am biased, because I work for them... but they still offer more trans-continental flight options between LAX and MCO throughout the day and night than any other carrier.
Have a great trip!