Our last trip we stayed for 11 nights at POR and then 3 nights at AKL. We did book through DreamsUnlimited and not Virgin, but it shouldn't make any difference really. If you contact Virgin holidays they should be able to change your booking and as you are increasing the cost of the holiday they shouldn't charge you any fees etc.
Not sure if you can actually upgrade your booking at POR, you may need to go through Guest Relations who will be able to contact Central Reservations for you.
Moving from one WDW resort to another is really easy. You get an automated bill on the day of checkout, so you don't even need to go to reception to checkout. You arrange with bell services at POR to collect your bags and transport them to your new resort. You can then go off to enjoy a day in the parks. You then just leave the parks and go to your new resort. Check in as normal and arrange for bell services to transport your bags (that usually arrive at your new resort by 3pm) to you new room
