If they are monitored contacts, the easiest way is to implement the pop-up for contacts with opt-out status. By displaying such a pop-up, we are able to effectively encourage the contact to re-write.

If we tag contacts with opt-out status on an ongoing basis, we can display them on the website a personalized banner with redirection to the landnig page, where the registration form will be displayed. 
We can also use the webpush channel, social media and mobile marketing for this (the contact does not need to be monitored for the last two)

We do not recommend a one-time e-mail sending to contacts with opt-out status due to the possibility of many spam reports, and thus weakening the reputation of the domain and mailing ip.