I don’t believe there is any legislation which specifically prohibits you from sending your blackpowder proofed revolver to the proof house to be reproofed to nitro - many have had firearms reproofed from blackpowder to nitro. HOWEVER the proofmaster may not accept a firearm for proofing if he considers that it will put him and his assistants in “unusual danger”. The consequences of using nitro in firearms intended for blackpowder are well documented. I believe that the revolver cylinders made by the likes of Westlake and Anvil for nitro loads are specially made for nitro, rather than modified blackpowder proofed cylinders, and I do not doubt that they would have exercised, and have been able to demonstrate, diligence to the proof master prior to submission for testing. Personally I would not risk using nitro in a blackpowder revolver - i value my hands, face, friends, etc too much. I’d also question how your friend has managed to get authorisation to use the revolver on his own land - vermin control?!