Auto-Populate Fields if Blank

There are several fields in my table that are often "N/A". I would like these fields to auto-populate with "N/A" is blank or to be able to have a button that will turn these fields value to "N/A" when clicked. 

I see there is the "default value" feature, however this doesn't seem to work on records that have already been created. For this use case, the records are already loaded into the table with basic information and the customer is responsible for filling in the details, so the "N/A" doesn't populate on those already-created records.

What is the best way to do this?