There is no change in verse 17 from Matthew 5:17, but there are interesting changes in the next verse.
Matthew 5:18: “For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.”
3 Nephi 12:18: “For verily I say unto you, one jot nor one tittle hath not passed away from the law, but in me it hath all been fulfilled.”
In 3 Nephi, the Matthean phrase “Till heaven and earth pass” is deleted. The final phrase is altered from “till all be fulfilled,” to “but in me it hath all been fulfilled.” The difference is the distinction between the mortal Jesus and the resurrected Jesus. In His mortal realm, that fulfillment was still in the future; by the time Christ appeared to the Nephites, it had already been accomplished. Therefore, there was no reason to declare that the law of Moses would continue until it was fulfilled, which is what earlier Book of Mormon prophets had taught, because (also as prophecied) Jesus was that fulfillment.
In Jerusalem, the people were interested in whether or not what Jesus taught contradicted the law under which they lived. He affirmed that the law was valuable, but that there would come a point where it would be ultimately fulfilled.
The people gathered in Bountiful would have had the same questions about the relationship of Christ’s doctrine to what they had known. Jesus sets the stage for teaching them the important differences by declaring that the prophesied fulfillment had come.