4" cast iron waste pipe

I was browsing this forum and a question arose about my waste pipe. i have the old 4" cast iron waste pipe (w/ lead in each connection) going into my basement floor. My tub, sink and toilet drain in to it. I have the old cast clean out and can not get it open to clean. It still drains fine but i know it hasn't been cleaned for years. i am going to gut my bathroom soon and want to go pvc on all my drains. I would also like to get rid of all the cast iron pipe. This main cast iron pipe goes all the way up into my attic and through the roof. My question is can I cut out 3-4 sections of the main cast iron pipe in the basement so that I can replace it with 4" pvc. If I take these sections out will the remaining cast iron pipe above come crashing down without support until I get the new pvc pipe in? Do you need to support it? Thanks for your input!

 

Start at the top and work your way down when you remove the pipe. That way there is nothing above you to come crashing down (And it will!!) When replacing it with PVC (white) or ABS (black), start at the bottom and work up.

 

I wasn't going to replace the vent portion of the pipe because I would have to repair the roof if I took it all the way out. I'm also not sure how easy it will be reaching the pipe within the wall. I can get to it in the attic but the attic to the basement might be fun. I was just going to replace the sections in the basement. But, if the pipe is not suspended and will fall if I take out the lower sections it would be easier to replace the whole pipe. whats the best way to dismantle the pipe. Sawzall ? Thanks!