what 2017 NEC code talks about knob and tube replacement
Although what Billy says may be technically true, that doesn't mean it is not a good idea to update it. A lot of insurance company's won't insure a house with known knob and tube wiring.  The insurance company execs are not stupid.  It has nothing to do with code compliance.  They know that all k&amp;t wiring is 70 to 80-years old and never designed to handle the loads of the modern-day house hold.   Stick with your pitch.  They don't need chapter and verse.  They just want someone who will upgrade it for a reasonable price. Good Luck. P.