During the fifties one of the most popular ways to keep homes heated, particularly in apartment buildings was home steam heating systems.
These steam heating setups work in a quite simple way, and there are fairly straightforward comparisons to be made with forced hot water heating systems. Like the hot water systems, steam central heating units require a boiler. In the case of steam, it obviously heats the water to steam point.
The advantage of working with steam is that it does not require such a powerful pump to set the steam moved around - this is why it way chosen for apartment blocks. There are other important differences from hydronic radiant floor heating systems and the like that mean it is often not worthwhile switching from steam to forced water.
Because the heat is delivered as steam, the pipes are larger and differently angled from forced hot water heating systems. Delivery pipes are insulated and the narrower return pipes are not - this is because the steam should return to the boiler as water.
For these reasons you are unlikely to find an easy and worthwhile way to switch from steam to forced water heating. When the time comes to upgrade, you might want to think about a complete overhaul and go for the most energy efficient system you can find.