Andrew Ford this is an interesting read: http://www.bbc.co.uk/news/technology-44240664 & what I talked about with Eliza Jane Fyfe. I am not sure you really need to re-consent users of your list to allow you to continue to send newsletters to them. The law cannot be retrospective and assuming you have collected the data in a reasonable way (sounds like you have, as you ask people if they wanted your newsletter), protect the data properly (using say a two factor authentication on your mailchimp list + strong password) and are using it to send information that is relevant to why they signed up (so about piano lesson related activities…not about cheap holiday packages to Spain) then you are still complying with GDPR. I have a feeling that many of the big companies are simply trying to cover their backs by “re-consenting” their users, however, I have now seen several articles that say this process is unnecessary if data is being reasonable handled for reasonable purposes. In addition, if the first port of call, if someone would have a complaint, would be for them to unsubscribe, contact you to highlight their displeasure at receiving it. I believe, from reading, at worst you would get a warning, however, I think you could demonstrate that this newsletter was relevant to the users subscribed and you always gave them the option to opt out.