Issue: For a Single Server configured with a public "postgres.database.azure.com" (FQDN) address, what migration path to select for a Flexible Server that would also end up needing a public "postgres.database.azure.com" (FQDN) address?
Answer: If both are publicly accessed as you mentioned, so no further action needed at networking level.
Hope that answers your questions.
Regards,
Oury