How Type 23s became general-purpose frigates

Were general-purpose Type 23s built differently from anti-submarine Type 23s?

No. They began as the same anti-submarine design; some were later changed by removing the towed-array capability to reduce costs.

The Royal Navy did not order wholly different Type 23 subtypes. The ships were built to a common anti-submarine basis, then some had their towed-array equipment and associated capability removed to make continued service more affordable. HMS Montrose is the concrete example of a ship built for ASW before later serving in the general-purpose group. That conversion explains how the Navy pursued forward deployment without creating a new frigate class.