There is no 8" pipe anywhere in that picture. Did you mean 3"?
Do yourself a favor and get rid of every bit of copper drain and vent pipe that you can. If you don't, you will have to do it later.
You will want to cut the cast iron off just below the hub, and use a transition band to convert to PVC.
Honestly, unless you are really handy and already have a bunch of tools, you would probably be ahead to have a plumber do this for you.