Apple Numbers wants you to use the Numbers format, so they automatically convert the file for you.
The quickest way I know (and do) is use Finder to click on the CSV file. Numbers opens. Do my thing. Then Export to CSV, but instead of making a new CSV, save it on top of the old one and replace it. The file name will be grayed-out, but point to it anyway so you don’t have re-type the file name, and then do it.