Why apparently simple warship upgrades can fail

Could French pre-dreadnoughts simply have been up-gunned or converted?

Not economically: construction quality and structural limits made major changes riskier and more expensive than new ships.

An apparently simple up-gunning proposal depends on the ship matching its drawings and calculated margins. Steel quality, plate thickness, riveting, welding, and fabrication tolerances determine how safely a hull can absorb new loads. Small imperfections become more serious when armament or structure changes after construction. In the Danton-class case, creating a dreadnought-style armament would have cost more than scrapping the ships and building replacements.