What has been some of your guy's experience?
Working in the Twin Cities of Minnesota, I have experienced a lot of different "jurisdictions." In my area, your question is understood to not be answerable from the
NEC. Rather, one must look to any local ordinance that covers maintenance to existing dwelling wiring systems.
Most of the jurisdictions I work in don't have any local ordinance governing existing dwelling electrical system maintenance, therefore, the only rules are that of the
NEC. And that is, simply, one rebuilds each K&T surface penetration to current
NEC requirements, nothing more. If the K&T includes a K&T fuse box, then, yes, replace it, but otherwise, one is only working on a branch circuit and it's more modern overcurrent protective device.
When the walls are not opened, per room, to 50% or more exposed framing, one simply rewires each existing K&T receptacle, light, or switch location complying with the current
NEC. The end result is an increase in safety, even though the number of receptacles, lights and switches has not changed. This is seen, by the Authority Having Jurisdiction, as good.