We have cruised the PG twice if Feb., and each time she carried about 200 guests out of a 320 guest max. double occupancy. As was stated above, if you book a guarantee, you may get an upgrade. Also, some credit cards, TA's and TA consortiums offer upgrades in the deal.
But in the absence of any of these factors, if you book a port hole cabin, that is what you'll get regardless of whether the ship isn't full. But wait, you still have one chance. Under these circumstances we have negotiated an upgraded cabin on Radisson for a price less than if we had originally booked it.
Hope this helps,