I'm about 50% finished with the code to let ships upgrade to the next class at colonies. Actually, that's not completely accurate, since it is possible right now for a ship to land at a colony and issue the Upgrade command and go up to the next class level.
Of course, this is wide open right now, and any ship can land at any colony and issue the order successfully. Now I'll have to make it actually require some type of construction materials and currency, and restrict it to only being allowed at a colony with a shipyard of the appropriate level that is owned by the same player.
In order for ships to be allowed to upgrade at a colony not owned by the player, I plan to have it so that colony governors can allow any ship to upgrade at a set price or a governor can issue Upgrade Authorizations which will allow a ship to upgrade to a set level at a set price.
So a governor at a colony with a Shipyard Level 2.4 would be able to issue an Upgrade Authorization that would allow shipID #2324 to upgrade to level 2.0 at a cost of 200 stellars per 0.1 level. Or you could allow a player to upgrade any of his ships to a certain level at a certain cost.
Now that I read this again, I'm probably only about 25% done with Ship Upgrades. Heh.