No, you wouldn't need to "reset" all users, they can be migrated over. Each user would need to create an Adobe ID, Enterprise ID or Federated ID for the Marketing Cloud (if they don't already have one). Once created, their Adobe ID would be linked to their solution login.
Customer Care can simply change a setting within your DTM account that restricts login to the Marketing Cloud. This essentially turns off the solution logins (dtm.adobe.com) for your account. Users would need to login to the Marketing Cloud (marketing.adobe.com) then navigate to DTM.
If your organization also has access to Analytics, Target, and Audience Manager, those solutions can also be linked to the Marketing Cloud (if they're not already). Once your solutions are linked appropriately, you'd have a single location to manage users and permissions for all your Adobe marketing solutions.
This is Adobe's vision for user management. You'll find each of the solutions is at various stages of integration with this vision. Some will have full functionality in the Admin Console, while others will only have user management. Eventually, all solutions will be fully enabled for user and permissions management via the Marketing Cloud
