My Home Depot Wiring 1-2-3 manual (2000) says the following about DIY re-wiring projects:
It's possible that some of the wiring in your home fails to conform to current regulations. That's usually not a problem as long as it conforms to the rules that existed when the wiring was installed. But any new work, even if it connects to old work, must meet code. (p. 52, emphasis added)
I'm wondering if there are specific criteria in the NEC for determining "old work" vs. "new work." If not, during a re-wiring project, how do professional electricians or inspectors decide what must be changed and what may be left as-is?
(Guidance, such as "if you touch it you must update it" seem too vague to address specific situations. For example, My upstairs was professionally re-wired two years ago to replace tube-and-knob with Romex. Outlets, receptacles, and boxes were also updated. Yet the number of receptacles and distance between them was not changed, even though that aspect is not consistent with modern NEC. Perhaps that was a mistake on the electrician's part. Perhaps not. If I understood the criteria professionals use to make those kinds of determinations, I could try to apply those principles to other situations in my own work.)