I have seem some references to the reverse of this but not what I am asking for. Probably it can be done but I am missing it.
I have a schematic of about 100 parts and want to use the renumber feature starting with-say-500. And then of course I need to renumber the PCB in parallel. (Just not in the mood for renumbering 100 parts!)
(The above will be melded with a similar circuit with a similar numbering scheme starting at 1).
Their similar, but not identical. Pcbnew / Tools / Load Netlist reads a text file from disk. Pcbnew / Tools / Update PCB from Schematic does not use the intermediate file on disk, but directly takes data from the schematic or Eeschema.
The direct method is newer, and usually more convenient, but the older way from the netlist file can be useful still. For example, some like to remove the GND and Vcc nets on the first import to reduce ratsnest lines during component placement. Especially on 4 or more layer boards with dedicated power planes.