Problems with the improvement of the fleet (Stellaris)

I decided to create a cool diverse fleet. Several types of corvettes, destroyers, etc. and when it came to improving our fleet is the following situation:

For example I created two drawings of corvettes: one attacking and one torpedo. It is clear that the torpedo military power more than the attacker, since the laser and the disruptor, and torpedo, respectively, the torpedo and disruptor. All body kits on the ships masthead. In the game there are new levels, guns, and fair like the Emperor, who cares about his fleet, I started to improve the kits. And when they put the fleet on the improvements in the spaceport, all the ships were improved not only in one kind, i.e. all corvettes had improved to the torpedo, and the destroyers, too, until the ship, which is the most power in their class. How can I fix this? Improved to Attacking to attacking, etc., but not to were you tried to create every model ship in the class, and then the bastard AI has improved every one of the supposedly most powerful model?!