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migrate wpf 4.0 application to wpf 4.5 to required also upgrade expression blend 4 to expression blend 4.5.
or expression blend 4 is ok with new Migrated WPF 4.5 Application.
There is a migration guide available on MSDN here: http://msdn.microsoft.com/en-us/library/ff657133(v=vs.110).aspx
Blend for Visual Studio 2012 with Update 2 supports WPF applications targeting the .NET Framework 4.5.
