Migration from Jira Service Management to HelpDesk
There is no particular scenario of migration between Jira Service Management and HelpDesk or vice versa. Those are different products and they are not connected: HelpDesk doesn't use the Jira Service Management information and doesn't extend it, neither do Service Management. Both Jira Service Management and HelpDesk extend the same core Jira functionality and projects.
When you deactivate/uninstall Jira Service Management, you can still keep the projects with their basic configurations (issue types, workflows, etc.). You will not be able to see and use such functionality as SLA, queues, portals, request types, organizations, automation, etc. though.
Jira Settings → Applications: here you can uninstall the product but keep the configuration.
Here is a view of the issue after uninstalling Service Management. As an admin, you can change the project type of this project to Business or Software and select this project to be displayed via HelpDesk.