Can you change a PRV by isolating the radiator at both ends?
A hot water heating system has to have enough water pressure in the heating system (boiler, piping, and radiation) to keep the water up to the highest point in the system. So, for each pound of water pressure in the system, the water will rise up in the piping a height of 28" or 2.31feet. For example, if the height of the highest point in the heating system from an accurate pressure gauge on the boiler was 20 feet, the boiler static pressure or cold fill would have to be 8.66# (20 divided by 2.31). Most PRV settings are 12psi for a 1 story house and 15 psi for a 2 story house. So, if you remove the TRV there is pressure in the system that is being supplied by a PRV set at at least 12 psi If you want to remove any of the isolation valves you should reduce the pressure to a point that the water will not flow out of that pipe. MYself, I would drain the system.  Hope this answers your question.