I am gonna post it here, because it sort of is a question.
So I am still on v1.51, because I wanted to get farther than 5yr into a game finally. That is why I do not even dare to post in bugs for now. I finally got to designing a new gen of Imp. NPE, with up to date subtechs for engines. Naturally, that means a class refit for me too. Since this is early game I was gonna change my Geosurvey ships a bit. And I did, though NOT AS PLANNED.
Updated the design how I wanted it to be, with my fears being whether I will need to retool SY. Turns out the ships updated too. They now fly faster and I can see in the design window they are the new design. I originally planned to keep the old and obsolete it by making a copy of the old one and modifying that one, but I did that, pushed update armour and noticed weight went down (despite no new armor tech) so I deleted the copy design, went back to original to investigate and forgot to make a copy again. I got the Unlock design prompt but if my memory serves me right VB did not allow you to modify a design of ship that was in service. Here I did just that and the ships are updated instantly without needing refit.
So sorry for being lazy, maybe this was fixed, if yes ignore it, though if not Steve should probably learn of this. I put in bold what I believe is/was a bug.