If the glacier ice breaks off the sheet and floats in to the ocean, it automatically raises the sea level at that moment...When it melts, it only the volume it has already maintained...Sea rise would = ZERO....
I believe the concern is for ice that is sitting on land, not ice that is floating on water. If this ice melts and its water travels to sea, it could increase the ocean volume. Big if. If it just freezes somewhere else, we do not have a net change.