Plumber or bust! I think I can do this! Advice please!

I'd like to try do DYI on a new kitchen sink and half bath that I've posted about b4

The #1 issue right now is cutting out some cast and connecting PVC there is old cast all on the horizontal run. I'd rather do the new hook ups to PVC

The 2nd floor stack I am leaving. it looks good in the chase coming down and it works well
plus I am not confident I can tear it out and re-do the 2nd floor bath I'm not confident to do that. But the stuff I would connect to PVC is pretty simple I feel I can
do it after getting advice on how to vent it from the forum experts.

I would Firnco PVC to Cast and this seems pretty easy. I also understand cutting cast can be tricky. The stack from the 2nd floor in the pic is not supported as far as I can see inside the wall coming down but the horizontal portion it is tied into has 2 U supports hung from floor joists on either side right near the connection. I may try to support the stack if I can in the wall it comes down in a chase and I removed the face not much room in there though.

if anyone has an caution or two and plumber experience advice that I should be on the lookout for I'd sure appreciate it. I am not going to try this until I am confident
I can indeed do it.

Thanks!

Your drawing shows the "easy" part of the job. Now you have to decide HOW you are going to install the lav and kitchen piping properly. Of all sad words of mice and men, the saddest are these, "What a bummer! Terry said I should have called a plumber".

Be VERY careful of cutting out CI with leaving stuff above...it is VERY heavy. It is built from the bottom up, with it all being supported like the branches of a tree. Cut out the trunk, and the top can fall, or may shift. You need to ensure it is well braced and can't move. Plus, I've heard it won't pass code with pvc underneath the CI...check on that.