Sewer drain replacement

I had an issue with my basement sewer piping under my slab basement and now I'm replacing part of it... as I am familiar, anything buried needs to be 4" SCH40-PVC or ABS. The down-drain leading from a bathroom to the 4" pipe is 3" cast iron pipe. Currently, the 3" cast iron meets a Fernco sleeve about a foot off of the floor. I was going to just cut off the rest of the 3" pipe and replace it with a 3 to 4" fernco reducer to PVC.

1. Is there an advantage to removing the old vertical cast...

Sewer drain replacement