How many feet of baseboard are there on each zone? It matters- here's why:
The minimum fire output of the NCB240 is something like 17,000 BTU/hr, so for zone radiation output to balance with the boiler output it needs to be able to emit something like 17,000 BTU/hr. The
temperature of the water makes a huge difference in the amount of heat emitted. If the boiler is putting out signficantly more heat than the radiation can emit, it rises in temperature until it either balances or it hit's the programmed high-limit, losing efficiency to either non-condensing temperatures or rapid cycling (or both.)
In order to get ANY condensing efficiency out of the NCB240 the water returning from radiation has to be below 125F, (average water temp of maybe 130F) and to break into the mid-90s for efficiency it needs to be below 115F, which means an average water temp of something like 120F. Fin tube basboard typically delivers 200-225 BTU/hr per foot of length @ AWT= 120F, , 250-300BTU/hr per foot @ 130F AWT, so to balance against 17,000 BTU/hr you'll need something like 70-80' of fin tube
per zone for it to run in condensing mode without short-cycling. You can cheat that a bit if the thing can be programmed for a significant temperature difference between high/low limit (I haven't read the manual to see what the options are, but you should before accepting that as a solution.)
So, do at least the napkin-math on the radiation & output. If it's possible to add either radiation or enough thermal mass that the minimum burn times are at least 3 minutes at min-fire you'll do OK. (I'll save the thermal mass napkin-math for later, and only if you think you're close enough on the amount of radiation at least for the upstairs zone.)
The total design heat load matters too. If you have gas-use history with the old boiler we can use fuel-use against heating degree days to determine a fairly firm upper bound on the heat load. A mid-winter gas bill with both the fuel use and the EXACT meter reading dates it covers, a zip code (for weather & outside design temp data), and the input BTU and D.O.E. output BTU of the old boiler would be enough. If you used auxilliary heating (wood stoves, etc.) or were off on vacation for the gas billing period, pick a different bill.