Here the editors for the 1920 LDS edition changed the when to then, thus eliminating a sentence fragment. The original manuscript undoubtedly read when here since both the 1830 edition and the printer’s manuscript have when. One could argue that the original text had then and that the scribe in 𝓞 (presumably Oliver Cowdery) misheard Joseph Smith’s then as when. Both words are identical except for the acoustically similar voiced continuant at the beginning of the word. Yet it is worth noting that although such a theoretical mix-up is possible, we have no specific evidence elsewhere in the manuscripts where when and then have ever been switched, even momentarily.
In favor of the 1920 emendation, one could note that later on in 3 Nephi there are other places where Christ quotes a biblical passage (sometimes extensively) that he says will be fulfilled. Each of these passages is introduced by a clause that begins with then, and in each case we get the inverted subject-verb word order (where the finite verb precedes the subject):
Thus here in 3 Nephi 16:17, one could argue that the 1920 emendation represents the original text.
Although this emendation seems reasonable enough, David Calabro (personal communication) argues that when is actually correct. In this passage, Jesus starts to refer to the future fulfillment of a prophecy of Isaiah’s (namely, Isaiah 52:8–10), which he then quotes. But after quoting the passage, Jesus does not provide any commentary on it. In fact, he cuts it off abruptly, ending up with a sentence fragment (“and when the words of the prophet Isaiah shall be fulfilled which saith : thy watchmen shall lift up the voice … and all the ends of the earth shall see the salvation of God”). Jesus, perceiving that his audience has become weak and is no longer able to follow his discourse, decides to put off his explanation of this quotation and to send the people home right then (as explained at the beginning of the next chapter, in 3 Nephi 17:1–4). But then Jesus changes his mind a second time when he has compassion on the people and decides to stay for a little while longer (as explained in 3 Nephi 17:5–8). For the question of Jesus’s apparent lack of omniscience in his dealings with these people, also see the discussion under 3 Nephi 19:26.
In support of this analysis, Calabro points out that on the next day, when Jesus comes back, he eventually returns to his previous discourse. This time he does not quote the verses from Isaiah 52. He already did that on the previous day—and besides, the Nephites can read Isaiah’s prophecies since they are recorded. Jesus nonetheless refers to the fact that on the day before he had not explicated that passage from Isaiah 52:8–10, and he twice uses the same subordinate conjunction when in reference to the fulfillment of that prophecy:
Here Jesus explicitly says he is now going to finish what the Father had earlier commanded him to say about those words of Isaiah. Jesus even reminds his audience that he already quoted the words to them (“ye remember that I spake unto you and said that when the words of Isaiah should be fulfilled”). By repeating his earlier language (but not the Isaiah quotation), Jesus once more uses the word when. Thus the use of when earlier in 3 Nephi 16:17 is almost certainly correct. The resulting fragment in the text is actually intended, and the critical text will restore the when to the text, despite its difficulty.
Summary: Restore in 3 Nephi 16:17 the original subordinate conjunction when, the reading of both 𝓟 and the 1830 edition; at this point Jesus intends to expound on the fulfillment of Isaiah 52:8–10, which he is about to quote, but after quoting the passage Jesus decides to save his explication for a later time; textually, the resulting fragment is definitely intended.