Merge pull request #107986 from mimckitt/patch-95

MicrosoftDocs/azure-docs#50063

Author: PRMerger6

articles/virtual-machine-scale-sets/virtual-machine-scale-sets-upgrade-scale-set.md

@@ -338,6 +338,7 @@ Some properties can only be set when you create the scale set. These properties
 - Availability Zones
 - Image reference publisher
 - Image reference offer
+- Image reference sku
 - Managed OS disk storage account type
 
 ### Properties that can only be changed based on the current value