Pretty sure that in ANH each of the hero X-Wings was used to represent another ship on screen more often than not, with the possible exception being Red 5, but I'm not even sure about that one, since it was apparently their favorite for beauty passes.
So yes, it's probably fair to say that these miniatures were interchangeable to the filmmakers from the beginning.
In ESB they only needed one of the original hero models for all of their shots, so they chose Red 3. I'd like to think they selected that model because of the partially destructive surgery they needed to perform in several places in order to add landing gear for the Dagobah swamp scenes. But that's just one guess.