How do you change baseboard heater to cast iron radiator
I presume this is a forced-hot-water system?

The question is going to be whether the room was originally set up with a radiator, or if the plumbing was redone for the baseboard. 

If you're lucky it was the former, and  the hot and return pipes come out relatively near to each other (with a return running the whole length of the baseboard to connect back to that area). In that case you drain and disconnect the baseboard loop, install whatever plumbing and valves are needed to connect the pipe ends to the two ends of your radiator, refill the system (including venting the air out of the radiator), and it should basically Just Work. The main challenge, outside of hauling the radiator up there, will be making the connections; if the new radiator isn't exactly the same size as the one that was originally there you'll need some additional pipe, creatively arranged, to make at least one of the connections work out properly. (My library was retrofit that way at some point; the radiator there now is clearly wider than the original and both ends are connected via some additional copper tubing.)

If you're unlucky, and the baseboard's return is at the far end from its supply... I don't think there's an easy fix for that case. You could run pipe all the way across that wall, but that may not save you much over just leaving the baseboard in place. You could re-plumb the return, but unless there's some other nearby part of the return to hook into, easily accessed, that starts turning into a much larger renovation project. I suppose you could leave most of the baseboard in place and use it as the connection to the return, giving you both radiator AND baseboard active in that room... though a good radiator might release most of the heat before it ever got to the baseboard.

I presume you've already checked this plumbing detail, and that you're happy with where the radiator would wind up.

(I periodically think about replacing the radiator in my living room with one of the sculptural radiators a few companies are making. That'd be a relatively easy retrofit since it's ground floor and I'd have easy access to the pipes. It might mean losing some of the period charm of the place, but having a piece of "functional artwork" appeals to my engineering instincts.)