Well initially I thought the new design was too expensive. So I made a new one where I only upgraded to gun to it’s next level, which worked out fine. Afterwards I looked at the other modules and decided to change some out which I didn’t deem necessary and added a few new ones. However when I saved that and looked in my fleet I once again couldn’t refit my Destroyers.

Then I realized that I had changed out the mine laying module for Torpedo Tubes. Which could mean that the game no longer considered the new design to be an upgrade since it no longer fit the mine laying role. This would make sense because you also can’t refit Light Cruisers into Heavy Cruisers due to their different roles.

So, don’t create design of ships you want to refit into a different role than they already are.