I asked my TA about this and she said we can upgrade for $12 per person per day (plus gratuities); it's definitely cheaper to do it BEFORE you go. I know people are saying not all Premium drinks are actually available, but I think it'll be worth it for us anyway, considering Premium also includes freshly pressed orange juice (which we love), premium coffees (I always find the regular coffee on cruises pretty disgusting) and waters (which we drink by the gallon on cruises!), so I think it'll be worth it for us.