Replacing cast iron piping

luna12

I have a Monoflo system from the 50s and I will be replacing the boiler with a Weil-McLain CGa-6 cast iron boiler. I will be keeping the Monoflo piping and radiators, but I will make some changes to the near-boiler piping. I'll be using black steel pipe and iron fittings. I'm trying to anticipate any problems I may have beforehand, so I can be more prepared when I do the install. One thing that concerns me is getting the old cast iron piping apart. I have never worked on these particular pipes, so I don't know if they'll come apart, if they'll be seized, if the threads will be usable, etc.

To anyone who has a lot of experience taking this type of piping apart, do they generally come apart easily? Are the threads usually clean enough to reuse with black steel pipe?

My other question then is, if i do have to end up cutting the pipe, how could I mend it or connect it with new piping? What are some techniques/methods that can be used? From the research I've done, it doesn't seem that I'd be able to easily cut and re-thread them. And I haven't really found any sort of compression couplings or sleeves that seem like a durable, leak-proof fix.