Why moving a capital-ship turret changes the whole design

Could a Queen Elizabeth-derived battlecruiser move a turret from the after position to an amidships position?

Not as a simple variant: moving the turret would force a redesign of the ship's internal arrangement and hull form.

A battlecruiser derived from the Queen Elizabeth design could delete a turret to create machinery space, but it could not casually move that turret elsewhere. Relocating a heavy turret forces major structural and internal rearrangement. Those changes propagate through the hull form and turn the proposal into a substantially new ship. Gun layouts cannot be separated from machinery and structural design. The scale of the redesign also explains why the Royal Navy might treat the result as a new class rather than an ordinary variant.