Importing Updated Records

I have a database with 17,500 records. I have updated several hundred records in my original CSV file overtime as well as added some new records and columns. If I use Import Records and import my updated CSV matching the records to an existing column, will it update all modified records as long as the headers are the same? Or will it only import the new records and columns?

If not, how can I update my records without having to delete all and upload CSV as new object?

I suppose I could just try this, but it seems to take a while to import such a large CSV so I hope to know in advance before spending the time to import only to find out existing records weren't updated and only the new ones were added.