This is possible today in Omni with adjusting permissions and leveraging groups you can preserve their access and manage in bulk rather than individual levels.
Suggestions based on our current settings:
Update current permissioning defaults:
Content Sharing default: No Access
Connection default permissions: Viewer
Current users: construct user groups for old permissioning scheme for easier updating in the future. Add users to the relevant permissions groups to ensure their access is correct. Ensure proper folders and documents have been shared with the relevant users and groups.
Current Content: Place in folders for easier bulk permissioning, adjust to organization wide permissions to No Access to ensure external users cannot see the reports.
New users: Provision a user account for them, and provide access to the individual piece(s) of content you want them to have access to