Add downgrade in faq

Author: joexumsft

articles/azure-app-configuration/faq.yml

@@ -105,8 +105,17 @@ sections:
         answer: |
           You can upgrade an App Configuration store at any time, for example, from the Free tier to the Standard or Premium tier, or from the Standard tier to the Premium tier.
           
-          You can't downgrade an App Configuration store, for example, from the Premium tier to the Standard tier, or from the Standard tier to the Free tier. However, you can create a new store in the desired tier and then [import configuration data into that store](howto-import-export-data.md).
-          
+          You can downgrade an App Configuration store from the Premium tier to the Standard tier. For other tiers, you can create a new store in the desired tier and then [import configuration data into that store](howto-import-export-data.md).
+
+      - question: What can I do if fail to downgrade an App Configuration store?
+          You can view the error message in the response for details about the downgrade failure.
+
+          - **The target SKU does not support downgrade.**: The target SKU is not eligible for downgrade.
+
+          - **The total key-value storage size exceeds the limit of the target SKU.**: The total key-value storage size has exceeded the allocated limit for the target SKU. To free up space, consider deleting any unnecessary key-value entries. You can monitor your current key-value storage usage through the Azure Metrics dashboard.
+
+          - **The total snapshot storage size exceeds the limit of the target SKU.**: The total snapshot storage size has exceeded the allocated limit for the target SKU. To free up space, consider deleting any unnecessary snapshot entries. You can monitor your current snapshot storage usage through the Azure Metrics dashboard.
+        
       - question: Where does data stored in App Configuration reside? 
         answer: |
           Customer data stored in App Configuration reside in the region where the customer's App Configuration store was created. Customer data is replicated to another region only if the customer enables [geo-replication](./concept-geo-replication.md) for that region. This applies to all available regions. Customers may move, copy, or access their data from any location globally.