In Gal Civ II, it was possible to change your mind whilst a ship was into building phase.

You just had to change the ship type and all the construction points used for the previous ship was transferred to build the next one.

i.e : if you were building a Colony ship costing 100 points and you were changing your mind at 50% of building process for a Constructor Ship costing 120 points, the 50 points already used was transferred to the constructor ship. Thus, Constructor ship was needing only 70 points to be built.

In Gal Civ III, it looks like you have to wait the end of the 1st building process before starting a new ship. If I am correct, this is weird.