In the 1950's, our house was built with a Zinsco main service panel that ultimately served six two-wire (ungrounded) and two grounded (via AC/BX) circuits. In the 1990's, a new service panel was fitted elsewhere and the branch wiring from the old panel was extended to the new panel; the old panel is now a junction box.
Can I replace this old panel/junction box with a new sub panel and meet 2020 code compliance?
My foremost concern is whether this is a modification to the branch circuit wiring for the eight circuits mentioned above and as such would require bringing all those circuits up to modern code. I can add AFCI/GFCI breakers in the panel to address AFCI and ungrounded circuit requirements, respectively. But two of the circuits serve the Kitchen receptacles in a manner not consistent with modern code; there is one 15 amp circuit serving countertop receptacles, and another 20 amp circuit serving another countertop, microwave, nook, refrigerator, stove, and hood vent. Obviously that's one heavily loaded 20 amp circuit, and is part of the motivation to fitting a subpanel over by the kitchen so I have panel space for more dedicated kitchen circuits when I remodel in 10 years.
Another, more immediate motivation, to fitting a subpanel here is so that I can use the 50 amp feeder that serves the detached garage subpanel and passes through this junction box as the feeder for this panel. I want to add a 50 amp car charger circuit to the garage, and since I can't send a dedicated circuit in addition to a feeder, I have to abandon the existing feeder and run a 100 amp feeder to the garage instead. Fitting a subpanel at the junction box lets me utilize this abandoned feeder, and also makes more room in my main panel. Win-win-win!