You will need to submit a ticket in order to have a REDCap Administrator enable a project as Longintudinal after a project has been moved to Production mode. REDCap users cannot enable Longitudinal Projects after the project is in Production mode.
If you need to make several changes, you can request the project be moved by to Development mode temporarily.
For more information on Longitudinal Projects, watch this: https://vimeo.com/758113689