I think the square type have 2 holes to mount which maybe further apart than the diameter of the round ones? You would have to make a bigger hole for the rear of the round lamp for the bulb holder protruding.
Best thing is to remove a square one, they are both the same, just the lens differ and see if a round lamp will cover the 'mess' left by the square lamp. The studs in the square lamps are moulded into the plastic case and will probably spin when you try to undo the rusted nuts. I attacked the plastic inside with a die grinder to release the plastic box then cut the studs off with a grinder, someone holding the stud with a pair of molegrips so it doesn't snatch and you rip a groove in your panel.