The old things are related to the Law of Moses, and Jesus declares that they have “passed away.” The nature of their passing is not a removal, but rather a fulfillment. They will pass not because they are torn down, but because they are “filled up,” or completed. Indeed, while we might speak of them as having “passed away,” they are rather transformed. They still are the Law, but are transformed into something new as Jesus presents the Law of the gospel.
Jesus explains that since it was he who gave the Law, he is able to fulfill it. What he gave them, he can fulfill and transform. The specific Law of Moses is therefore at an end for the Nephites, and the full Law of the Gospel is in force. Of course there is much of the Nephite understanding of God that was already presaging this change, so there would have been little in the Gospel that would be surprising to them. The effect of the change would have been on ritual practice rather than individual religious understanding or devotion.