I agree... with a stipulation.
If I am building a Small attack craft that takes 6 turns to build and I have one turn left until it is complete. I should not be able to switch it to a colony ship that costs 7 turns and complete it in two turns...
Different ship styles require retooling. you should lose something.
However if I have "Harbinger of Death Mk1" building and I go and upgrade the design and make "Harbinger of Death MKII" I really think the shipyard should automatically update what it is building and build the new design...