The Book of Genesis does not mention Moses but in the middle 3 books Moses as the main protagonist and prophet is referred to in third person. In Deuteronomy however Moses speaks in first person mainly. Why the switch?
Technically, Deuteronomy is written in third person.
- The first five verses are in third person, ending with "Moses began to expound this law, saying:"
- Moses speaks from chapter 2 through chapter 30, and the main narration begins again with chapter 31 with occasional dialogue
- Moses recites a poem/song in chapter 32:1-43, and then the narration begins again in verse 44
- In chapter 33, Moses blesses each tribe of Israel, and each blessing has a narrative, third person header.
- Chapter 34 is all third person narration
So, to sum up, Deuteronomy is technically written in third person, but it seems like first person because it records Moses' last words.
Deutoronomy or Debarim (The Words) is, what Moses spoke to Israel towards the end of their travels through the wildernis and deserts of Arabia, and it were his final words to them before he died and before they entered the land.
So much of it is direct speech it would have been unnatural, had he spoken of himself in third person after all these (almost 40 years). Moses - I believe - was not in danger anymore that he would transgress by exalting himself the way it had happened when he and Aaron had exalted themselves before Israel at Meriba.
I don't agree with all the statements in the question, and I don't believe that there is a big switch.
First of all, consider this example as a starting point. Let's imagine that we are researching Martin Luther King's famous "I have a dream" speech. We can easily imagine different renditions of the speech:
- I have a dream... We might read the exact words that King spoke, if we had access to his sermon notes. They are in the first person because of course King himself wrote them for his own use.
- Yesterday Reverend King preached a magnificent sermon on racial justice. Here's the complete text of his speech: "I have a dream..." We might be reading a pamphlet handed out by King's supporters to reinforce his message. Here the pamphlet is in the 3rd person, but within that is the exact text, set in the 1st person because it's quoting King who was of course speaking in the 1st person.
- Yesterday Reverend King preached a passionate sermon at the Washington monument, in which he spoke of the dream he had for a home in which his children were able to experience true racial equality... We might read words like this in a newspaper report. The report doesn't quote King's words, but gives a general summary of the speech and its contents. Here King's message is in the 3rd person because it's part of the overall report set in the 3rd person. (King spoke and said that he had a dream...)
Now let's consider Moses' story. I think we can without too much trouble see how this basic idea applies.
- First person:
This form of writing doesn't apply in any of the first five books.
- Third person incorporating first person quotes:
Clearly this describes Deuteronomy. No matter what opinion we have about the authorship of the Torah, in its final form Deuteronomy is a writing where Moses' words are quoted in the first person but set in a third person framework. James Dunn in his answer gives a good outline of that framework.
But it's important to see that exactly the same writing style is used elsewhere. It's not limited to Deuteronomy. Consider for example Exodus 20, which records preparations for receiving the Law. Overall it is written as a third person account. But many of the spoken parts are recorded as direct first person speech. God's words in particular are all expressed in the first person. (See verses 3-6, 9-13, 21-2, 24.) And of course, the 10 commands follow the same pattern: A third person framework for the first person commands.
- Third person reporting:
Throughout the Torah, we see this pattern, where for example Moses' words are not quoted directly but are summarised. This too happens in Exodus 20: See for example verses 20-25:
- The Lord calls Moses to the top of the mountain. Actual words not recorded.
- God tells Moses to prepare the people. Actual words are stated.
- Moses reminds God the people cannot come up the mountain because it is holy. Actual words spoken. (Perhaps as a signal of the importance of those words about God's holiness.)
- God sends Moses down the mountain to prepare the people (v24). Actual words spoken.
- Moses went down and told the people. Here that single word is third person reporting. Its meaning is defined entirely by the quoted words of God in the previous verses.
So: Is there a switch from the earlier books when we get to Deuteronomy? I don't believe so. It's certainly true that the first person language is more extensive than in other books, but that's only because Moses' sermon is more extensive. There are more words.
As a final picture of the similarities, read the parallel versions of the 10 commandments in Exodus 20 and Deuteronomy 5. You will see clearly that while some details are different, the overall structure is identical. In Exodus there is a narrator who quotes what God said, and those words are first person. In Deuteronomy there is another level, because the narrator quotes Moses quoting God. but other that that, the commands themselves are all in the first person. The only changes are those necessary to reflect the slightly different perspective of the storyline.
Well there are parts in Deuteronomy where moses explicitly says "I" not just his last words. Here is an example; Deuteronomy 10:10: "I, moreover, stayed on the mountain forty days and forty nights like the first time, and the Lord listened to me that time also; the Lord was not willing to destroy you."