There are a few problems with changing the shiptype after passing the dock:
1. The game can´t be sure you really have passed a dock this turn. Maybe you are in the middle of the sea and wants to change your shiptype. No (easy) way for the game to control you have the right to change into a new shiptype
2. Changing into a new shiptype at the dock normally means your movecosts change as well, so maybe you would not reach the tile you are now standing on (with your new ship) if your shiptype had been changed at the dock.
I guess we can work around all this by allowing a ship to change shiptype anywhere, but thats not as much fun, and also not very logical to me