Replace cast iron stack.

Clutchcargo

I've got a cast iron stack that I considering replacing with plastic.
I'm redesigning/renovating the bathrooms and kitchen. Both will be larger room sizes and fixture locations will move. It's a two story house with bathrooms stacked and the kitchen immediately next to the stack.
The picture is the second floor bathroom floor joist. As you can see, the original location of that wye fitting from the tub and sink required a huge amount to be hogged from the structure and as a result caused the floor to sag. I'll be replacing all the floor joists. To avoid adding huge holes to the joists on the renovation, I want to lower all the 2nd floor plumbing so that the it hangs just below the joists.
I know that many will say why replace the cast iron with an inferior product but from my POV, I see this cast iron stack as a complete pain in the butt that's going to end up costing much more in the long run.
Am I wrong in my thinking and reasons to replace the iron with PVC or is tying into the cast pipe and replacing (lowering) the wye no big deal?

Edit, that 2" copper on the right is a separate vent that that makes a very circuitous route from the basement...I will also want to tie that into the main stack.

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TheEplumber

Your doing the right thing. The disadvantage of plastic is the additional noise the drain will make. The advantage is long term life.
If you recycle that copper- you'll probably pay for all the plastic pipe

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