A 5.9-inch Hipper would not become a viable light cruiser

Would replacing Hipper's forward 8-inch turret with two quadruple 5.9-inch mounts make her a useful light cruiser?

No; eight 5.9-inch guns would still be outmatched by twelve-gun light cruisers and 8-inch heavy cruisers, while Hipper remained too large to evade capital ships.

The ship would lose the striking power that justified its size without gaining enough barrels to dominate light cruisers. It would face the worst of both worlds: inadequate firepower against peers and a hull large enough for Renown or Repulse to catch and destroy.