Well, even if that were the case, who's to say when that time restriction starts given you could in theory keep moving your flight time as space opened up on earlier flights?

The last time I did this was at ARN, when I saved MrsNWI and myself a mind-numbing 5-6 hours in the airport by moving flights. Even though the Gold line offloaded us from our original flight and got us on the revised one, on boarding we (along with several others) were upgraded to CE because of large scale overbooking in Y - so actually it's probably even possible to change even if there's just space somewhere on the plane, not just in Y, which somewhat increases your chances.