Why a nuclear Great Eastern would cease to be the same ship

Could SS Great Eastern be retrofitted with a nuclear reactor?

Only through a rebuild so extensive that the result would no longer meaningfully be the original Great Eastern.

The panel rejects a reactor retrofit as a normal modernization. A nuclear plant would demand new machinery spaces, shielding, controls, auxiliary systems, and extensive structural alteration. So much of Great Eastern would have to be replaced that the project becomes a Ship of Theseus problem rather than a simple repowering. The watch point is useful for distinguishing an upgrade from reconstruction at the level of the vessel's identity.