Update migration-from-v3-to-v4.md

docs/migration-from-v3-to-v4.md

@@ -179,3 +179,28 @@ For model cmdlet, a parameter is required only if it is marked as required, both
 ### How to mitigate the breaking changes of x-ms-mutability
 
 There is no mitigation since it is actually a fix for existing issues.
+
+## None-fixed array
+
+In v3, if a response contained a property with an array type, it was generated as a fixed-size C# array that couldn't support adding new elements through functions like Add. In v4, we generate a list instead of an array to support the Add function in such cases.
+
+### How to mitigate the breaking changes of non-fixed array
+
+This is a design change and we do not provide a way to generate fixed array anymore. From users' perspective, the following modifications should be made in scripts that utilize PowerShell modules generated with v4:
+
+- Use Count instead of Length to retrieve the number of elements
+- Use the ToArray function if conversion to a fixed-size array is needed
+
+## Managed Identity Best Practice Alignment
+
+In v3, IdentityType and UserAssignedIdentity are generated and totally matches with swagger definition. In v4, we generate them to align our best practice design. As a result, generated cmdlets will have changes as below.
+
+- For IdentityType, it will be changed to EnableSystemAssignedIdentity\<SwitchParameter\> for new-* cmdlet and EnableSystemAssignedIdentity\<bool\> for update-* cmdlet.
+- For UserAssignedIdentity, it will be changed from UserAssignedIdentity\<HashTable\> to UserAssignedIdentity\<string\[\]\>.
+
+
+### How to mitigate the breaking changes of Managed Identity Best Practice Alignment
+
+It's recommended to migrate to best practice design for managed identity features. To avoid breaking changes in regular release, 
+- For IdentityType, configure `disable-transform-identity-type` as `true` in README.md.
+- For UserAssignedIdentity, configure `flatten-userassignedidentity` as `false` in README.md.