Steam heat is a matter of timing, the vents on the side of the radiator provide that timing. You might swap some of the upstairs vents with the downstairs vents to see if that might help. It is the hole in the vent that permits air to leave the radiator at a given rate. The faster the air leaves, the faster the radiator will get steam inside.
Vents come in different venting sizes to adjust the timing – some are adjustable, others are automatic. You will need to find vents with A,B,C,D,E, or 1,2,3,4,5 sizes to make the venting time longer in the upstairs rooms or slower in the downstairs rooms, or a combination until they all heat evenly.
It is possible, but less likely that the radiators upstairs are actually too small for the rooms, You could test this by removing the downstairs vents and plugging the holes. Running the system would force the upstairs units to vent until all the upstairs rooms felt comfortable. If one room never gets warm enough, you would know you need a larger radiator.
Exactly sizing the radiator to the heat loss of the room makes the process easier upon first installation, but after 50 years, insulation and room sizes change. You can find more information under ‘Steam Tips’ at http://www.hydronic.net