: Can't cut the cast iron with a cutting torch? OK, so would have to cut it with metal saw, which would be a pain. Therefore I should just rpelace the affected section with plastic?

: : You cannot cut it with a cutting torch. All you will do is melt it and have an end that will not be usable with any type of joint device.

: : : I'm moving some plumbing aorund underneath my house and have shorten some of the sewer pipes. Should I just cut down the old cast iron and reuse it (yes, have cutting torch available) or should I replace the full length with plastic?

Karl,
Plastic works fine. Over time, iron will rust out. They make banded couplings to convert between the two pipes.
Older cast iron, may be smaller than the current no-hub variety. I find that a copper to cast banded coupling works well in that situation.
Terry