An item's existing metadata is updated automatically in ArcGIS Version 10 by default. Information that is added automatically does not include the Sync attribute. This causes the Upgrade button to not appear in the Description tab even though the item's original 9.3.1 FGDC (Federal Geographic Data Committee) metadata has not been upgraded.
Force metadata to be upgraded for ArcGIS by opening the Upgrade Metadata tool from the Search dialog box and using it to upgrade Version 9.3.1 metadata. Use the appropriate Upgrade Type depending on which metadata editor was used to create metadata in ArcGIS Desktop 9.3.1. This tool can be run for all items at once by opening it in batch mode from the ArcToolbox window. The Upgrade Metadata tool is in the Conversion toolbox in the Metadata toolset.