Q class might still be "open" for people who qualify for it.

Let me try to give one example. Let's say Q class requires a 14 day advance purchase when you are flying from LAX to DFW. 13 days before your flight you try to buy a Q class ticket but the computer will not allow you to do so becuase you are under 14 days before departure so it puts you into a different fare bucket. Now, lets say a person started a trip from DFW(on a Q class fare) and was currently in LAX and was scheduled to fly back to DFW the next day but wanted to try to change the return date to a the following day. The fare rules allow you to change the date as long as you pay a change fee and you can book the new return back into Q class.

The other example would be a fare from LAX to MIA in Q class might only require a 3 day advance purchase---not a 14 day.