We use GF for membership sign-up and renewal. When a member renews, we use a form with the User-Registration add-on set to "Update User". When the form is used, the person who used it is added to the list of form entries. If they attempt to renew again, the form will not allow them to do so because their data is in the list of entries. The need to update a user multiple times is not uncommon. Is there a work around besides deleting the entries from the form?
You can use an Update User feed multiple times to update a user. Be sure you do not have a restriction like “no duplicates” on the email address or username field in the form. That would prevent multiple submissions, even without a User Registration feed.
You could actually disable the User Registration feeds temporarily to be sure the problem is with the form, and not the User Registration add-on.