O Fallon, MO
I can only speak knowledgeably of electrical code.
It depends on where you are at. In general, any modification where the "building finish" is removed, you'll probably be doing code upgrade.
Some municipalities go beyond this. In one place I service, K&T and aluminum wiring are not allowed. Period. The city will not allow a home sale to proceed until it has been rewired. Is this legal? IMO, no. Is it still in practice? Yes. I've seen several home sales fall though because of this. I've also seen people walk away from their houses because of this.
In most other cities, K&T and AL wire must be replaced where exposed. Some call it the "light of day" rule. Once it sees the light of day, it needs to be addressed.
In a gut rehab or major renovation, it must all be removed.
These aren't an NEC requirement, but a requirement of the municipality. It's basically enforced common sense. You would be an idiot not to rewire in those circumstances, yet there are people who go out of their way not to. Typically "flippers".
Up until 2011, the code didn't really address "upgrades". In 210.12 (AFCI stuff) it basically says if you modify an existing circuit where AFCI is now required by code, you must bring the entire circuit to AFCI. This is for residential.
There is some legal contention regarding this rule, as it is possible the NEC has overstepped its boundary. Almost all codes of past have taken a "grandfather" approach over the years. The existing structure is grandfathered to the code that was in force when it was built.
Just an example, a house built in 1960 obviously wouldn't pass the requirements of today. But we cannot realistically bring a house up to new standards every time code is revised. We have to raze every house built over six years ago and start over.
If someone refers to herself / himself as a "guru", they probably aren't.