Armor coverage and subdivision matter more than a battlecruiser's label

Would a thoroughly modernized HMS Hood become a fast battleship?

No. Converting Hood would require major improvements in subdivision and armor coverage, and the battlecruiser-to-fast-battleship boundary depends on more than belt thickness.

Turning a cruiser-derived protection scheme into battleship-level survivability requires extensive internal work alongside added armor. The Kongos form a spectrum: some gained enough subdivision to cross the line more convincingly than others. Armor thickness alone does not settle the issue because coverage and internal compartmentation matter as much.