Can you convert hot water baseboard heat to radiators? How hard? Would it be steam or hot water?
You should be able to remove the baseboards and install cast iron radiators keeping the hot water pumps and boiler if the system, especially the pipes, are sized appropriately. It might require new piping and it can be difficult to find good looking matching cast iron radiators of the right size and even harder to find someone who knows how to size them for modern systems.
Converting to steam would be a total replacement and there aren't many companies that install steam systems in residences anymore.
Sure, you can replace baseboard with radiators, panel radiators etc. No need for a steam boiler and piping.
Some links that may be useful.
You can convert from baseboard to radiators fairly easily. It will require repiping above the floor as the radiators two pipes will be closer together. Some floor repair will be needed and wall repair if you choose to recess the radiators. There also might be cause to put larger circulator pumps in as there will be a larger volume of water to move.
New cast iron hot water radiators (and perhaps steam, too, though I haven't looked for those) can still be purchased. There's a supplier in Brooklyn and a couple of others can be found on the web. If you can't turn up anything by googling, I will see what I can find in my saved files.
They are expensive if one choses the beautiful ornate Victorian-style ones, but plainer, less expnsive ones can be had. The suppliers can help with sizing.
It's always tempting to try and reuse salvaged ones, but they can have problems of their own that require a skill person to solve. There are always vendors on the net (including some firms that have completely rehabbed old ones) with some for sale.
Hot-water radiators are wonderful to live with. Their only disadvantage is their size, but the heat they provide is very cozy, in my experience.
Radiators can often be found at scrap yards for not a lot of money. Often they require lots of elbow grease to rehab them, though.
Steam heat systems can use hot water radiators, but hot water systems can't use steam radiators.
Fortunately, by about 1890 or so steam specific radiators had pretty much dropped out of production. It made no sense to make two different kinds of radiators when one would work on both systems.
When buy a cast iron radiator you should test it with air to see if it leaks unless you plan to take it apart and rebuild it.
In the old "column" type radiators (fat verticals) you need to be sure the tops of the sections are connected in order to use them for hot water. The newer "tube" type radiators (skinny verticals) are all connected across the top and can be used for both systems.
We used to pressure test radiators, chase out the tappings, clean them up and put them in storage. Other than rare and unusual ones, we've taken many tons of rads to the salvage yard, but the last time we needed some the selection was poor.
Is there a formula for how much (in feet) baseboard heating do I need for a specific size radiator? I have mostly ten fin cast iron radiators in each room but do not know how many feet of baseboard pipe I need per room.