if you mean a free upgrade there's no one here that's been able to figure it out. They are fairly rare. No ones mentioned getting upgraded now for a couple of week.
If you want to pay for an upgrade, you can do it any time before you leave at the "normal" rate.
If you want to pay for an upgrade at the port, you will be able to only if there is a room to be upgraded to. Have heard that you sometime don't pay as much for those upgrades. But they are also hard to come by as the ships usually sail near capacity.