Does every new warship class replace an older one?

Should Dido and Atlanta be understood mainly as replacements for older cruiser classes?

No. They were also products of fleet expansion and changing operational needs, so no one-for-one predecessor is required.

A new warship class need not have one direct predecessor. A navy may buy a ship to expand total strength, answer a new threat, or reorganize how the fleet fights. Dido and Atlanta emerged while their navies needed more screening capacity in the face of approaching war. Their procurement therefore added a new concentration of command and dual-purpose firepower rather than merely exchanging an old hull for a new one. Reading construction only as replacement can conceal the force-structure problem the design was intended to solve.