Technically true, but... it is a real hassle for an airline to do a last minute change of gauge-- everyone has to get new seat assignments, and invariably some people will get split up, lose their seating preference, etc. If the aircraft actually used is smaller than the originally planned aircraft (say, the pax load ends up being significantly less than expected, etc.), some of the people who have been upgraded may have to be downgraded (it's not unusual for the F cabin to be full of upgrades on domestic flights while the Y cabin load factor is extemely low).

Also, in many cases, the gate will have to be changed because not all gates can accommodate all a/c types, and this has spillover effects on other a/c operations.