Why the Spearfish Guppy was a different aircraft

Why was the Spearfish unsuitable as an airborne-early-warning aircraft?

Its original airframe did not suit the AEW role, and the Guppy conversion became an essentially new, unsuccessful aircraft.

Visual resemblance did not mean structural continuity. The original Spearfish airframe did not provide a sound basis for airborne early warning. Fairey's Guppy proposal retained a similar external outline while changing the internal airframe so extensively that it was effectively a new aircraft. Its rejection shows how a role conversion can cross the line from adaptation into an unsuccessful clean-sheet design.