Why anti-ship capability would not change Type 42 employment

How would dual-purpose surface-to-air and anti-ship missiles change the way Type 42 destroyers were used?

Almost not at all; they would remain air-defense pickets, but might run out of missiles sooner if the magazine served both roles.

The ship's assigned purpose governs its employment. The Type 42 remains an air-defense picket even if its missiles can also attack surface targets. A secondary anti-ship capability does not remove the force's need for the destroyer to protect the group against aircraft. Using missiles in the new role would mainly risk exhausting the same limited magazine earlier.