In a house that has two sets of Edison fuse boxes on the upper floors. Could the fuse boxes be live? If they update to 200 amps do you just bypass the old boxes or replace them with circuit breakers?
You would have to test them. They can certainly be live with the fuses still acting as OCPDs. Take the cover off and see what is happening.
They may have run new 14/12 guage wire and used to fuse box as a junction box or just re-connected the existing fuse box feeder to the new 200amp main.
The hard part is updating older services , and having to address what is often old cloth seu 3 wire feeders to modern 4 wire isolated neutrals