Why a stretched River-class OPV is not a cheap frigate

Which country could most easily turn a stretched Batch 2 River class into a frigate?

None; the design is a good OPV but a poor and uneconomic frigate basis.

The River class is designed for affordable presence and maritime-security work, with straightforward maintenance. A frigate conversion would require cramming in equipment and rearranging much of the hull. Apparent open space in illustrations does not establish that the ship can accept the proposed additions. The later follow-up returns to the cost of refitting the Rivers and the case for a purpose-built frigate.