It just means that the only economy discomfort seats available are the "Preferred" seats (which have no extra legroom except if it is an exit row). If no regular seats open up before your flight to switch into you will be assigned one at the gate. I once had 30C on a 75S and the Gate Agent made a call in the gate area that she had exit row seats available. I told her I would even take a middle (the only options were middle and window in Row 36) if there was no extra charge. Needless to say that opened up a regular seat for someone else (I think the Gate Agent was happy to have an aisle seat as all of them were booked before I made the switch)