Hi Jason,
A previous version of our developer guide has a section dedicated to the migration from 10.2.x to 100.x versions of Runtime. The last version of the guide with that section was released with 100.9. You can access the developer guide download through the dashboard. I can't link to the 100.9 guide download directly, so be sure to scroll down to select that version. https://developers.arcgis.com/downloads/#net
That document covers various conceptual differences, and also provides a table that maps the old namespaces and API names to the new names.
For graphics specifically, that functionality has moved to GraphicsOverlay. GrahicsOverlays are added to MapView and SceneView, rather than Map and Scenes; the guide section on migration goes into more detail about those concepts.
One more thing: the preferred approach for referencing the Runtime API is via the nuget package, ideally using the PackageReference format (rather than the old packages.config format). I understand that the nuget package includes some compatibility checks as well as other aspects of build configuration that may be missed with a manual direct reference to the contained DLL. This is a change from the documented process for referencing the ArcGIS Runtime API since 10.2.7.
Good luck with the migration and please do let us know if you have any more questions.