Rather than exporting the existing items to a CSV, updating that and then uploading, I think you need to click on the Download Template button in the top right corner of the Data Import page. This will download a CSV file that contains certain header information and where records begin at a certain line. Take the data from your exported file and copy paste it into the template at correct line, which says “Start entering data below this line”.
I used the items template in my screen shots, that’s just because I happened to be doing an import of my own at the time. I’m sure item prices has a similar template.