As the upstairs is cold, there is a lack of flow or lack of baseboard there, so check the temperature drop across the baseboard – if it is greater than 20F from one end of the baseboard to the other, the flow rate is too slow and the baseboard has some type of design, clog, or operating problem (like pressure too low in system creating an air trap.) It may just be that there is not enough baseboard in the rooms upstairs, so you can do a heat loss estimate and baseboard comparison for free at http://www.hydronic.net
If there is little insulation in the attic, a good cure is to add insulation to modern standards, then there might not be a need to change the system.
The flow to each baseboard or radiation upstairs depends upon water flow rate through the main, so check the temperature drop from the boiler and back to the boiler to make sure it is not greater than 20F – if it is, a circulator with a little more flow may be necessary.
Some possible cures for the system:
1. install non-electric radiation valves on the downstairs radiation while leaving the upstairs as is. By turning the downstairs radiation controls to maintain desired temperatures, the heating rate downstairs will even out while lengthening the circulation time to the units upstairs before the thermostat shuts the burner off.
2. Run the circulator all the time (it does extend circulator life for very little cost) and permit the burner to go on and off with the thermostat. This would extend the heating time upstairs.
3. A combination of the two above.
4. If the upstairs and downstairs are on separate circuits, zone valves in the circuits controlled by a thermostat on each floor can help.