Hi @Fred Thank you for posting your Question on Microsoft Q&A and sorry for the delay in response to your question.
In managed service you don't have a way to switch underlying persistence provider. If you are using FHIR Service in Healthcare APIs (currently in Public Preview), then you are already using SQL in the backend). But in managed service you don't have access to the underlying database. In OSS you need to redeploy the FHIR Server with SQL option.
In the future we will have ways to migrate from Azure Api for FHIR to azure healthcare APIs. If you want to do it now, you will have to reload.
Hope that help
Regards,
Oury