How an off-the-shelf frigate becomes a new design

Why has construction of the Constellation-class frigate been delayed?

Because repeated requirements changes transformed the selected proven design into something much closer to a new ship.

Constellation's delay is attributed here to the accumulation of changes to the selected frigate design, with Saudi construction rejected as the explanation. Buying an existing design initially promises less development risk, but successive modifications can remove much of its commonality with the starting ship. The comparison with a clean-sheet design is a cost argument: once changes become extensive, starting afresh could have been competitive. Familiarity can still make the modified project appear safer to decision-makers even after the technical work has changed substantially.