User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Other
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\general
This policy controls the ability of IT admins to initiate the migration of users from classic Outlook to new Outlook. If you enable this policy setting, IT admins will start the process to switch users from classic Outlook to new Outlook. If you disable this policy setting, the migration process to new Outlook will be stopped, keeping users on their current version of Outlook without transitioning to new Outlook. If you do not set this policy setting, the migration process to new Outlook will not start, and users that have not migrated will remain on classic Outlook. Note: IT admins can also define intervals for re-initiating the migration process for users who revert to classic Outlook from new Outlook. This is managed through the NewOutlookAutoMigrationRetryIntervals policy, offering a tailored strategy for transitioning users based on organizational requirements and user feedback.