As far as I know, asking them to confirm is not enough. You have to recollect the consent of each of them for the future use, and only resending the activation email is not enough, because the user does not “consent” explicitly to the use of their data and you don’t have this consent stored in your db.
An option could be the following:
0.- Adapt your register form to the GDPR with the new checkbox (it has to be unchecked), as explained here: https://beta.docs.mailpoet.com/article/252-how-to-add-a-checkbox-field-to-mailpoet-form
0.1.- (find a solution for the bug with the checkbox that I mention here: https://www.ads-software.com/support/topic/field-for-gdpr-acceptance/)
1.- Prepare a landing page with the form to register to the newsletter.
2.- Prepare a newsletter to ask all your users asking them to register again, with the new form.
3.- Send the newsletter and after sending delete all your subscribed users from the list.
The new subscribed users will to have to check the consent checkbox, and the consent will be stored by mailpoet in the local wp table (if we manage to find a solution for the bug mentioned in the other thread).
Yes I know what you are thinking: delete all my users? Well, in the case of a newsletter mailing list, as of 25 Mai you are not allowed to mail them if you don’t have the explicit consent. And if you didn’t had a checkbox in your form, then you do not have the consent.
Regards.