Just by simple editing of common\unit_upgrades file you can now really rebuild ships to newer models!
To test it I put there
And after just one year (instead of two) and mere 4.39 IC a day (instead of 8.7)I got brand new type of battleship. With just 30% of starting XP of course, but new and functioning nontheless. Finally we don't have to scrape that old junk anymore#Battleship
base = battleship
target = battleship