Technology changes and new stronger, lighter alloys are being produced—just look at the Conda.
Then every single ship would have a new variation because there's not a single ship in the galaxy that wouldn't benefit from reduced mass,specially the dedicated explroer ships. In fact, why would we actually reduce mass in warships first? There's no urgent need, reducing mass increases jump range, so if you did it first for warships that don't really benefit from increased jump range that would be rather strange.
And I see you've bought up the Conda despite me pointing out it's innapropriatnes to be used as part of this argument, we have had endless discussions with various people wanting nerfed to bring it in line with other ships or alternatively left alone so they can bask in thier uber-jumpy glory, it was a mistake on FDEV's part, just like Mitterands Hollow and has been left alone because people are used to it by now so it would break a lot of things, and people, to change it.
Lets stop with the trying to insert a rational explanation of why a ship should suddenly get reduced mass and admit you only want it for personal reasons and not game play reasons.
The answer is of course no, if I had new alloys and technology why the heck would I retrofit it to a couple of old out of date designs, clunkers that would never be able to use the new technology to best advantage, no I would just leave those alone and design a brand new ship and the old ships, just like transatlantic steamers, would be abandoned by all except pirates and poor people who couldn't afford the new shiny!