[Strategia_In_Ultima]

That would IMO be more of a retrofit than a ship-size upgrade. As it is, in SEIV (haven't had the chance to try the SEV demo yet, and don't know if I will anytime soon), you can refit a ship to any other design of the same size, provided the new design adds no colony modules or spaceyards. Retrofitting a Battlecruiser-size ship from a cargo transport to a heavy-hitting warship is also rather extreme.