One of our classes has updated their class list email addresses, street addresses, etc. I downloaded the .csv file for them to use and now want to upload the revised data. Can I save the new file as .csv and upload to replace the old info with the new? Will the system only update the new data? Almost 1/3 of the original list is changed. (the attached file is the updated version. Updates are in red)

Unfortunately, no. That upload feature is only meant for populating the data to the site for the first time and not meant for ongoing data maintenance. Any updates should be made directly to the profile records via the Manage Classmates > Enter / Edit Classmates area.