Does it have to show a shift in the author's thinking? Of course not; it could quite legitimately reflect a change in attitudes and approach to Jesus' teaching by the Jews over the course of his 3-year ministry. Remember that there is a similar shift in narrative in the other 2 synoptic gospels.
Rosenstock-Huessy seems to think it shows a shift in the author's thinking.
Here's the link again:
http://tinyurl.com/ppwtpk9He says, on page 23 of the book, that Matthew "writes from speaking as a Jew to speaking as a non-Jew" and that "by chapter 28... the Jews are no longer divided into believers [in Christ] and unbelievers in Christ. The Jews as Jews are outside Matthew's family. The fence between them and Matthew is infinitely higher in chapter 28 than in chapter 1.
The outpouring of his experiences, his memories, his notes, changed the writer's own mind. Everybody should become a different person by writing a book." (My brackets and emphasis added).
Then he says, "The wisdom of our tradition consists in the fact that in the first gospel a man writes himself out of Israel by writing up Jesus."
"... An evangelist is a man who, by speaking of Jesus, changes his own mind; by being in process, he leads others into the same process. The gospel of Matthew instituted the process of seeing the world and Israel in a new light because it was this very process itself."
"... Standing upright and pleading in danger of his own life, and then abandoning his own Jewish allegiance, Matthew wrote his gospel. He reversed the meaning of the Bible by experiencing that it was no longer the
last word. The last sentence of the gospel... expresses this fact very simply."