Allow me to paraphrase my previous post for clarity:
In my area, replacements must be built to code as a matter of professional integrity.. I've not yet been able to locate a specific code requirement to satisfy your request.
Simply put, I offered up nothing to the original conversation, and also never implicitly stated that the op was required to rebuild his stairs to code.
professional integrity? you'd tell someone to spend hundreds of thousands of dollars based on what?
your feelings? i'm a civil engineer, i am required to follow rules and codes, i've signed documents to that fact. if i told a client that they had to modify their home to bring the stairs up to code and they sued me, i'd be on the hook to pay that bill. any contractor would be on the hook.
For some items, you are correct. If you renovate an area and use the existing stairs where they are, even if not to code, you are ok. If you remove the stairs and install new ones, they must meet code. If you have a basement, then it does not have to meet code. If you renovate that basement and make bedrooms, then they must meet code, have fire egress windows, two means of exit, and smoke detection among other things
again, you are both offering up opinion and what you think is code. this is why simple electrical questions go off the rails in this forum. people offer up opinion as hard facts and then repeat that opinion as fact, when there are actual rules to follow.
as an electrical contractor i am required to know the code. ungrounded outlets are common in old houses, there is no code that requires you to upgrade the electrical to live in or sell a house. you are required to add gfic receptacles if you alter the wiring, like in a remodel, but no code requires me to tear into all the walls to upgrade to current code. believe it or not knob and tube wiring is still code. as are old stairs.
i agree that if you alter the design, like add bedrooms to a basement you need to meet updated egress and wiring requirements. but you have altered the design. just like a remodeled bathroom must meet new code. if you notice i've asked the op twice about the old stringer and received no reply.
there is also no code that requires anyone to gut their home and do structural changes when updating stairs. unless you can quote code, don't quote code. for me to rebuild my stairs to code would require deleting two bedrooms upstairs, losing 1/4 of my living room, adding a column in the center of living room and relocating the upstairs hall. i'd have to structurally rebuild my house, it would be less cost to tear it down and build new. tell me what code requires this.