I booked a "Guaranteed 1A" on the Canival Glory Last Year. That is the cheapest room avialable with an upper and a lower berth on the lowest deck on the ship.
To my suprise, they upgraded me to a Category 4B - an inside cabin one deck higher with 2 single beds pushed together into a king sized bed. If I had booked the room I got, it would have been an extra $300. This year I travelled on the Victory, I booked a cat 8A, and they gave me an instant upgrade to an 8C - my 4th trip with Carnival.
The upgrading procedures are done well in advance of boarding. I don't know the exact procedures they use, but reservation date - earlier the better, and previous cruises with Carnival are also factors.
Carnival does tend to look after its repeat customers, as thanks for your business
Our last three cruises have all been guarantee bookings. So far each has given us an upgrade.
Granted it is not always a huge upgrade, but if you book a guarantee at the lowest grade you would be happy with, then you have nothing to lose. FOr our Alaskan trip, we booked interior and they moved us to a Frenchdoor Oceanview. So, like you we got a good deal.
As far as what time to board, look down the Carnival forumn and read the thread on that subject. Titled "What's the best time to board."
We have sailed twice with Carnival booking guarentees with both. The first was a guarenteed balcony and we got upgraded to a Balcony Suite, we were surely spoiled then, it was FANTASTIC... last cruise we booked a guarenteed oceanview and might have gotten upgraded to a higher category oceanview but we booked so far in advace that our travel agent with Carnival practically guaranteed us a balcony upgrade which didn't happen when we recieved our tickets. Two weeks prior to cruising we called and paid the extra $400 to upgrade to a balcony and were glad that we did. My parents did have a nice oceanview stateroom up on the Spa Deck which had a HUGE panoramic window for them and they could see everywhere but I like balconies especially at night time and when I just want to get away from the family....
Been in that panoramic oceanview cabin. It is really nice. You have your own little table/sitting area in the window space that you don't get in a normal oceanview stateroom. Almost like a mini suite. The windows goes floor to ceiling/wall to wall. The only drawback is that you can't go out on the balcony (because it doesn't have one), but if you don't need that, its a great room.
I just haven;t found the need to do the balcony. Not worth the extra cost to me. Of course we have been sailing with our two girls lately. Once the wife and I are solo, it might be nice to have the balcony.