RE: What happens if I upload the same CSV again – or a subset of it – with some fields changed.
The rows will be imported as new entries.
RE: what information needs to stay the same so the system knows it’s the same entry (i.e. it can’t just be the name, maybe name and address.
Correct, it is not possible to “know” if the CSV record is new or an update with just a name or even a name and address.
To update an existing directory entry listing via CSV, you would add an “Entry ID” column to the CSV file and add the existing directory entry listing ID value to this column. This is the same process as plugins that allow importing and updating WordPress Pages/Posts (though some plugins enable you to choose which CSV field should be considered when determining a duplicate page/post [update existing by Post ID, Title, or Slug]).
RE: would it be better to export the current list, delete entries, update and re-import? That seems like a mess.
Actually, this is precisely what I recommend as it is very quick and easy to apply sorts and filtering to a CSV file, make changes, remove unchanged rows, and upload/import only the changed CSV rows, as that saves the time of creating and/or updating the previous CSV files with the entry ID value.
I hope this helps; please let me know.
