Fleet Air Arm aircraft as submarine hunters

Were the Firefly and Barracuda adapted for anti-submarine work only because the Swordfish needed replacement?

No; the Royal Navy deliberately expected almost every Fleet Air Arm type to carry the 100-pound anti-submarine bombs.

The Firefly and Barracuda did not acquire anti-submarine loads merely as emergency substitutes for the Swordfish. Royal Navy policy made almost every Fleet Air Arm aircraft capable of carrying the small anti-submarine bombs. That approach spread convoy and fleet-protection utility across the air group instead of confining it to one specialist type.