Modify MSBuild ItemGroup Metadata

There is a new to modify metadata by using the Update attribute E.g.

<ItemGroup>
    <Compile Update="somefile.cs">  // or Update="*.designer.cs"
        <MetadataKey>MetadataValue</MetadataKey>
    </Compile>
</ItemGroup>  

More in the MSBuild documentation

Yes you can modify or add to an <ItemGroup>'s meta data after it is defined (MSBuild 3.5)

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <!-- Define ItemGroup -->
  <ItemGroup>
    <TestItemGroup Include="filename.txt">
      <MyMetaData>Test meta data</MyMetaData>
    </TestItemGroup>
    <TestItemGroup Include="filename2.txt">
      <MyMetaData>Untouched</MyMetaData>
    </TestItemGroup>
  </ItemGroup>

  <Target Name="ModifyTestItemGroup" BeforeTargets="Build">
    <!-- Show me-->
    <Message Text="PRE:  %(TestItemGroup.Identity)  MyMetaData:%(TestItemGroup.MyMetaData)  OtherMetaData:%(TestItemGroup.OtherMetaData)" Importance="high" />

    <!-- Now change it - can only do it inside a target -->
    <ItemGroup>
      <TestItemGroup Condition="'%(TestItemGroup.MyMetaData)'=='Test meta data' AND 'AnotherCondition'=='AnotherCondition'">
        <MyMetaData>Well adjusted</MyMetaData>
        <OtherMetaData>New meta data</OtherMetaData>
      </TestItemGroup>
    </ItemGroup>

    <!-- Show me the changes -->
    <Message Text="POST: %(TestItemGroup.Identity)  MyMetaData:%(TestItemGroup.MyMetaData)  OtherMetaData:%(TestItemGroup.OtherMetaData)" Importance="high" />
  </Target>

  <Target Name="Build" />
</Project>

Reference: MSDN Library: New Methods for Manipulating Items and Properties (MSBuild)