If you enjoy the sound of toilets flushing inside your walls, get rid of the cast iron pipes and install plastic.

I'd ask the plumber about the state of your pipes, not us.

When I lived in our 1852 house and had our walls open, my Dad, an engineer, recommended we replace with PEX. And I did as my father suggested. I was a rebellious teenager and am now a totally compliant daughter. Yes sir engineer! ;)

Agree with @Joseph Corlett, LLC

The cast iron stack is fine unless it shows signs of rot. Rot will look like corrosion - usually on a wye. If you see anything iffy, try poking it hard with a screwdriver.

Cast iron is going to be quieter than PVC.

If you demo the stack, be very careful because it's crazy heavy and it will break. It should be obvious, but start at the top.

You want to get rid of all the galvanized pipe and all the copper at this point and start over. Galvanized will close up - both supply and drain lines.

A good electrician can snake a lot of wire without doing too much plaster damage. But yes, all of the wire needs to be replaced.

Are you keeping it a triplex? Food, Retail, and Office all have significantly higher electrical needs than the residential panels will likely supply. You are probably going to re-do the service entrance and panels also.

Since the property is going light commercial, you have to evaluate the needs regarding the electrical and water service to the property.

Keep some money aside for redoing the sewer lateral and the water supply lateral. If they are original, they are likely to be bad. You can have a local sewer cleaner run a camera down the sewer to tell you what condition that's in.

If you have a galvanized water lateral, plan on replacing it.

It will be a small bookstore/coffee shop on first floor, book storage in basement, my office on second floor, and the city won’t let me occupy the third floor (finished attic) without installing a second egress which I won’t.

My electrician has inspected the service and the three panels, she says I have enough power; if necessary I can put in a bigger panel for the first floor, if really necessary I can get a bigger service. The sewer line is 6” and I’ve had it scoped and CIPP lined, so we’re ok there. New water supply line.

I figure that with so much opened up, I want to take the opportunity to replace as much as needed before we close it up again. My electrician is a friend and giving me a friend rate, I promised to make the job as much like wiring up a new build as possible. (And I’m going to install the rebuilt transmission in her BMW for her.) I have no attachment to the original lath and plaster, happy to rip it out and drywall whenever needed!

On the other hand, my condo board just spent $250,000.00 on relining each of two 5-story stacks of 1973 cast iron drains and vents.

Hmm. I spent $8,000 to line about 60’ of sewer line.

Have you checked if you need to add fire suppression for the new uses in your jurisdiction? (not a terrible idea even if not required).

Galvanized supply definitely lines need to go-- they are usually fine until you touch them in any way and then they are not. Keep the cast iron stacks if they are in good shape; beware that they can rot from within and look fine on the outside but be paper thin. Your plumber should be able to help.

Strongly consider going with blueboard and plaster in an old building rather than drywall. It will look much better and much more appropriate to the structure. Provides a much stronger surface. I know it's location dependent, but cost is very similar where it's regarded as normal; much quicker to install and no sanding mess to deal with. (It's hard to find anyone who wants to do drywall where I live-- they all hate the multiple rounds of return trips for sanding)

I will look into blueboard and plaster! Thank you.

I do not have to add fire sprinkers. I was originally planning to do so anyway, but the cost would be nearly $50,000 because the Fire Marshal requires a separate 1.5" supply line just for the sprinkler. I asked sprinkler companies why this is, when our city code explicitly says all fire sprinklers may use a combined sprinkler/domestic water supply; they shrugged and said the Fire Marshall simply won't approve that, and there's no recourse.

I would like to use firecode drywall where-ever I do install drywall, for a bit of added fire safety.