Steam boilers are sized to the connection and the connection calculation consists of measuring each radiator to determine its EDR (Equivalent Direct Radiation).
Steam-only radiators may or may not touch across the tops of sections and then cannot be converted. Radiator sections needs to be coupled together on both bottom and top so air can be eliminated and water can flow between with the warmer water rising.
Converting each radiator requires you to drill out the upper vent and chase the threads, remove the steam trap and install in its place or remove the guts of the trap to permit water flow and replace the radiator valve.
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