I'm sorry to report this, but rebuilding checks were broken in present version (And maybe in other ones) Played at Windows 8.1, repeated with every in-game nation in every game. Options used: Super large fleets, variable techs, harsher peace, manual legacy fleet Problem: Ship under minor rebuild may lose or gain major design features, or can't be rebuilded at all. Examples: - Russian-built ABY dreadnought has central firing. I want to install British Director at her. It is impossible, as Brits didn't invented B turret. - German-built AV sent to US for CVL conversion. She lose 1 level of torpedo defense, but gained AON bonus. - US-built battlecruisers lost AON during upgrading their Directors in Germany To recreate bug: - Create ship with highest available torpedo defense, certain turrets, AON scheme, Director, or with DP guns, mines. - Try to blanc rebuild her at nation without some of these techs - Some features should disappear, others may make design illegal.