Why some Type 45 modernizations are harder

Why are the last Type 45 modernizations expected to be more difficult?

The remaining ships need an earlier design change incorporated before the wider modernization work, making the job more complex and costly.

Four Type 45 destroyers had already passed through the modernization sequence discussed in the stream. The two difficult hulls were left until later because they predate a design alteration incorporated into their sisters. That earlier change must first be rebuilt into them before the wider upgrade can proceed, increasing cost and complexity. A common class name can conceal production-batch differences that matter greatly during refits.