Almost. It was hijacked in an area that did not have primary radar coverage. A flight switch like you described could have been performed for AA77 (although there was really no need to have the new aircraft appear at the point of hijacking).
However the other three flights were hijacked in areas with primary radar coverage. This makes a switch exceedingly difficult. UA175, in particular, was hijacked in airspace while the ATC controller handling the flight had primary radar activated, and was actively searching for a non-squawking primary return. Making such a swap in that instance, without the controller noticing, is an impossibility (putting aside, for the moment, stealth technology).
-Gumboot