You can cast RefPtrs to base types, just like normal pointers.

Glib::RefPtr<Gtk::TreeStore> refStore = Gtk::TreeStore::create(columns);
Glib::RefPtr<Gtk::TreeModel> refModel = refStore;

This means that any method which takes a const Glib::RefPtr<BaseType> argument can also take a const Glib::RefPtr<DerivedType>. The cast is implicit, just as it would be for a normal pointer.

You can also cast to a derived type, but the syntax is a little different than with a normal pointer.

Glib::RefPtr<Gtk::TreeStore> refStore =
Glib::RefPtr<Gtk::TreeStore>::cast_dynamic(refModel);
Glib::RefPtr<Gtk::TreeStore> refStore2 =
Glib::RefPtr<Gtk::TreeStore>::cast_static(refModel);