A Type 31-derived air-defence ship is possible, but it would need much more than additional missile cells. Sensors, computer systems, and power generation would have to support the new mission. Research and development would also be needed to integrate that equipment. The result would lose much of the original light frigate's attraction as a cheap and quick ship to build.
Notable Point · 2:30:51-2:32:05
An air-defence Type 31 would be a much more demanding build
Turning Type 31 into an air-defence destroyer changes its combat systems, electrical demands, development work, and construction cost.