The only way I could find to change the .NET framework version for an existing project using VS2010 was to edit the *.vbproj
file.
From:
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
<TargetFrameworkProfile>Client</TargetFrameworkProfile>
```xml
To:
```xml
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
<TargetFrameworkProfile />