add public preview note

Author: duongau

articles/frontdoor/tier-migration.md

@@ -11,6 +11,9 @@ ms.author: duau
 
 # About Azure Front Door (classic) to Standard/Premium tier migration (Preview)
 
+> [!NOTE]
+> Migration capability for Azure Front Door is currently in Public Preview without an SLA and isn't recommended for production environments.
+
 Azure Front Door Standard and Premium tiers were released in March 2022 as the next generation content delivery network service. The newer tiers combine the capabilities of Azure Front Door (classic), Microsoft CDN (classic), and Web Application Firewall (WAF). With features such as Private Link integration, enhanced rules engine and advanced diagnostics you have the ability to secure and accelerate your web applications to bring a better experience to your customers.
 
 Azure recommends migrating to the newer tiers to benefit from the new features and improvements over the Classic tier. To help with the migration process, Azure Front Door provides a zero-downtime migration to migrate your workload from Azure Front Door (class) to either Standard or Premium tier.