There is a thread with known upgrade costs on these boards. They might help you figure out the answer. That being said, the chances of being able to upgrade to Concierge at port are slim to none. I would never base any decision on the slim chance of possibly upgrading to Concierge at port.
First, most cruises sail with all Concierge cabins booked, especially on 7-night cruises. The fact that Concierge deposits are non-refundable, albeit transferable, make those bookings much more stable than regular ones. Also, if someone cancels at the last minute, the newly available stateroom will be offered first to other Concierge guests in a lower category. So, if the Walt or Roy seems available 3 days out, it might not be once the terminal opens.
After that, the few guests doing a B2B will have first dibs on the remaining staterooms since they get to check-in around 9AM, usually at the same desk where the upgrades are managed. Managers have been known to “advertise” upgrades to guests doing B2B.
When all those possibilities have gone through, upgrades are usually taken in the first few minutes after the terminal opens, so you need to show up very early at the terminal to be first in line, or, if sailing out of Port Canaveral, be on the first two busses from MCO since people on them enter the terminal before it opens to guests arriving on their own.
