How to generate referral links for old customers?
By default, Refer a Friend plugin will generate referral links for every new user that registers with your site, while for already registered users it will be generated as soon as they view any page where their referral link is showing. But, if you need to generate it at once for all your past customers, maybe you want to send an email with a referral link to all your customers. Just go to WooCommerce -> Status -> Tools and click on "Create Referrals" button.
That is all. Now all your customers will have referral link generated for them.