As I understand it, the main difference between these two is that bimaxillary protrusion is primarily about the incisors jutting forward, while upswing is about the whole maxilla.
But swinging the maxilla forward would surely worsen the appearance of borderline bimaxillary protrusion.
So is this a real cause for concern, or is it somehow not a real issue?
Bimaxillary protrusion is caused by the tongue pushing up against the front teeth. If you have bimaxillary protrusion and simply just stop thrusting the tongue against the front teeth it will naturally fix itself from the force of the lips.

Take a look here, the only reason this girls teeth can tip forward so much is because the force of the tongue exceeds the force of the lips, it's as easy changing the position of the front tongue when swallowing and at rest, the lips will tilt the teeth back into proper alignment.