Some cast iron rads need the flow and return connected the correct way round; e.g. flow on left, return on right. This because there is a deliberate obstruction in the bottom 'pipe' which prevents the water taking the easy path, along the bottom between entry and exit. The rad installation instructions should tell you if this is the case. If so, just swap over the plastic pipes at one end. There is no need to change the TRV to the other end; it will work just as well fitted to the return. PS Bleeding rads should always be done with the pump off, and preferably when the water is cold. Start on ground floor and move upwards.