Cast Iron Radiators

How should I calculate how much baseboard heating surface/length I need? I am considering combining two smaller rooms into a master bedroom on the second floor. Each room has a small section of slant finn baseboard.

would you change/upgrade slant finn baseboard to cast iron baseboard?

I have cast iron baseboards and radiators on the main floor. The second floor/zone upstairs has slant finn baseboards. I might upgrade my old old fired boiler to a more efficient one. If I do would it make sense to upgrade the upstair baseboards at the same time?

Thanks!