VS 2017 does not allow you to completely change to the Project Format for Windows Forms projects.
However you can get rid the old package.config file and you will be able to partially use some of the nuget features and use the transitive nugets references.
Follow these steps:
- Remove all of your nuget packages manually
- then delete the packages.config file.
PackageReferencein your csproj.
After that the legacy project can use the PackageReference NuGet references and do transitive restores.
Notice that project-to-project references are not transitive and you nee to project-to-project references still.
If you are interested in migrating your WPF projects I think that this StackOverflow thread