if you take a headphone and remove the drivers and place them into new cups, would the sound change significantly ?
What if we change the drivers for given cups ?
It depends on the headphones and its drivers. Most of the times the frequency response change in a noticeable way if you make an important change of cups. But I think we can not generalize.
I've changed the stock cups on my Takstar HD2000 for MDF cups with a different shape. The sound has change significantly in the lower end.
I feel like I'm repeating what the others had said but it is both. If you don't have a good seal/drivers sitting farther away from cup you will have less bass presence than if they had a good seal and were placed closer to your ear. With my K272s and more notably HD600s removing the thin pad of foam that sits between the driver and your ear lessens the amount of bass and brings out the details of the treble more. I've also found that removing the grills of the HD 600s gives a much wider sound stage with very slightly improved imaging.
That being said not all mods or changes done to a headphone will be positive. Drivers made for closed cans will probably sound pretty bad in an open configuration (I know the same applies to open back headphone drivers working in a sealed environment, taking the grills off the HD600s and cupping your hands around the sides makes them sound absolutely terrible)
EDIT: I've also heard of the HD800 mod being pretty popular (though I have no experience with it)