Retargeting solution from .Net 4.0 to 4.5 - how to retarget the NuGet packages?

NuGet 2.1 offers a feature that makes this a lot simpler: just do update-package -reinstall -ignoreDependencies from the Package Manager Console.

NuGet 2.0 doesn't handle re-targeting your applications very well. In order to change your packages' target frameworks, you must uninstall and reinstall the packages (taking note of the packages you had installed so that you can reinstall each of them).

The reason packages must be uninstalled and reinstalled is:

• When installing a package, we determine the target framework of your project
• We then match that up with the package contents, finding the appropriate \lib\ folder (and \content\ folder)
• Assembly references are added with Hint Paths that point to the package's \lib\ folder, with the right subfolder (\lib\net40 for example)
• Content files are copied from the packages \content\ folder, with the right subfolder (\content\net40 for example)
• We record the targetFramework used to install the package within the packages.config file
• After you change your project's target framework, the Hint Paths still point to net40
• When you uninstall packages, we check the targetFramework that was recorded in packages.config to see what target framework's libs/content to remove from your project
• When you reinstall the package, we detect your updated target framework and reference/copy the right libs/content

For those who had problems with update-package -reinstall <packagename> command, consider running it with -ignoreDependencies flag, like this:

update-package -reinstall <packagename> -ignoreDependencies

This flag will leave your package dependencies alone, otherwise they might got updated even if the package you originally wanted reinstall still keeps it's version in same.

More info here.