One of the most annoying things when you finally get all the new toys is waiting years for your shipyards to finish retooling for them. Large civilian ships and most warships suffer from very long retooling times, which are especially noticeable in lower RP games when researching tech and engine of ship can take decades or longer, only to be stopped at the final step for even longer to retool shipyard.
My suggestion is simple. Allow retooling for prototype class, but not its construction.
This change would reward future planning and allowed for more streamlined transitions between generations of ships. As for in-game reasoning for such change: Even if all specifications of some techs are not known yet, general idea of how it will look and be integrated into ship should be known in advance, as well as many other aspects of ship that are already researched and are not going to change, which can be prepared before making finishing touches allowing for full use of new tech.
Another way this could be implemented would be to simply halt retooling at 90% for any design with prototype components and only resuming once all are researched.
