under slab plumbing

Could anyone please give me some advice; I'm planning to do a basement renovation in the next week or so , adding a shower and moving a toilet. I was wondering what I should do when I tie in to my sewer line which is under the concrete slab. Above slab it's abs pipe 3" and a secondary 2" line 4 inches
away( I'm assuming the 2" pipe connects to the main sewer line at some point
below surface) they both connect into cast iron which runs through the floor.
Should I replace both lines with abs pipe and run them through the concrete floor instead of having the old cast iron/ abs old connections or just tie into the cast iron further down the sewer line.
I hope this makes sense!!??
Thanks for any help

I would run abs to wherever you tie your new shower and move the toilet to. Use a service weight gasket if you have to connect into an existing hub or use a fernco if dealing with pipe to pipe.

We don't have ABS in my area anymore; stores quit selling it in 2000.