The old things (the law of Moses) have “passed away”—not by being removed or torn down but by being fulfilled or completed. Indeed, it might be more accurate to say that they have been transformed. They are still the law but are transformed into something new: the law of the gospel.
Yahweh-Messiah explains that, since he is the lawgiver, he has the right and power to fulfill and transform it. Of course, much of the Nephite understanding of God already presaged this change, so little in the gospel would have been surprising to them. The change would more likely affect ritual practices than individual religious understanding or devotions.