Reference: The most probable reference for this scripture is: “But many that are first shall be last; and the last shall be first” (Matt. 19:30). It would not, of course, have appeared on the brass plates nor is it recorded in Jesus’s sermons at Bountiful. As I read it, Joseph saw the text’s relevance to the situation Moroni explained and shaped his translation accordingly.
However, applying this scripture of the first/last reversal is difficult in this particular context. Typically, the phrase indicated the reversal between the house of Israel and the Gentiles. That is clearly not the case here. The context here is the separation from Israel and the reuniting of Israel in the two Jerusalems. Perhaps the existence of the two Jerusalems creates a first/last reversal in some way not clear to a modern audience.