[Mike Swearingen]

You can cut cast iron with a sawzall with metal cutting blade or a snap cutter made for the purpose, but whatever you use to cut it, make certain that you have braced the cast iron from dropping down when you cut it.
All of the weight of the cast iron above the cut will be on it. Use 2x4 bracing if necessary.
Good Luck!
Mike

 

[liquidplumber]

It is better to cut in a cast iron wye then transition to pvc....like Mike said make sure the pipe is supported before you cut it.

 

[dOe]

Thanks!

Usually though don't stacks have braces clamped on them between floors?

 

[oh'mike]

I know a plumber who used to think that all vents were anchored--(they should be)

He's short two fingers on his left hand now.

Always be cautious of other peoples work--Mike---