Heh. Good to know. We recommended to anyone who didn't want to have that happen create a new class "Argonaut Mk. 2/B" etc.
That is the preferred method when creating a Mk2 version of a ship. Bring up the ship, click Copy (at the bottom), modify it, rename and lock.
Yep, after you're done designing the class, you're supposed to lock it. Note that that makes the design tab go away
It also ensures that you don't have any design errors - if the button is greyed out (you can't lock it), then you should look in the panel to the right 'cuz something's probably wrong (e.g. not enough crew quarters). Note that you can push the same button to unlock the design - the locking is intended to protect you from accidentally changing a design that's bound to actual ships.
STEVE - I vote that unlocked classes give an error message if you try to refit a SY to build them, or if you try to build one in a SY. That would both prevent the "Why did my ship design change" error, and its partner "Why doesn't the class I just designed show up as a possibility for retooling" (which would start turning up if you just silently removed unlocked designs from the list).
John