We update records through an API. How can we set some sort of rule or permission that enables the API UPSERT function to overwrite an existing value IF the existing value is something specific (like NULL or NA), but NOT if it contains another value. (I remember being able to do something like this in SFDC... where I could specify, for instance, a certain user could update/create/push a value value as long as there wasn't already an existing value in the field.)

We are running into this issue, by the way, because "Last Name" is required, and we don't always have that value on the system we're syncing from so we just have to push something at all times, which is "NA" in our case. Fine for new records, but not fine for existing records that actually have real "Last Name" values. So that's fairly annoying.