Does the Customer Import allow for updates of current customers or only additions of new ones?

[Chasidy Spurgeon]

I activated the Customer Service Management feature and created fields like email address, company, phone number that we want to capture for each of our customers. We use the customer portal and I was expecting these fields to be available for the user to complete on their portal profile, but that does not seem to be an option. As an alternative, I am hoping to be able to bulk update these values from a CSV, but the Import Customers documentation and past questions don't address if the import allows for updates or if it will just try to create new accounts for our existing users. Has anyone had any experience with trying to use the Import Customers to make bulk changes? Does it work?