In going through my own boiler research, I stumbled into a friends home, which has steam radiators. He hooked up a modern oil boiler and simply converted to hydronic that way. I didn't ask how many pipes were changed, but he's using the old rads. He is extremely meticulous - the home is a registered landmark and is truly a museum.
They really like how it heats and said the biggest change was that there is A LOT of water in the system and he had to use 3 expansion tanks.