The solution is to create judges rather than kings. While Mosiah does give some reasons for this change, there is little enough definition to the changes that we may suppose that the concept of judges was not foreign to the people. Indeed, the most critical aspect of the judges is mentioned by Mosiah: “..to judge this people according to our law…”
It is impossible to have judges without law, for there would be nothing against which they could judge. A king does not need law because the king is the law. Thus in a kingdom, a new situation can easily be handled because the king makes a decision (presumed to be the will of God) and it is then law for all. The removal of the king removes that personal link to the will of God, and therefore something must become the basis for rule. That basis is law.
The Zarahemla society has already been working according to laws, and not simply the law of Moses. When there is persecution of the church, the issue is brought before Mosiah, to judge against the law. The result was the creation of a new law.
Nevertheless, it does appear that at least the mechanisms of law and judging are already in place for Zarahemla. Without having such a foundation, the simple declaration of this type of change in government would not be possible. The promotion of judges and the demotion of the king presupposes that something else would take the role of the king as the final judge. Here, it appears to have been law, a conception that was already in place.
Historical: Mosiah’s selection of the term judge for this new position was most likely informed by the brass plates and the history of the judges in Israel.
Interestingly, however, Israel moved from judges to a monarchy, while Mosiah is taking his people in a different direction. While the terminology is the same, there are some significant differences between the Israelite judges and the Nephite judges.
The Israelite judges appear to have begun as military leaders, and were never the true political leaders of their country, where the Nephite judges held the government and there is no hint of an overt military function (other than as required of a head of state). (see Tvedtnes, John A. “Kings and Judges in the Bible and the Book of Mormon.” In: The Most Correct Book. Cornerstone Publishing, 1999, p. 194-7 and Merrill, Byron R. “Government by the Voice of the People: A Witness and a Warning.” In: Mosiah, Salvation Only Through Christ. Religious Studies Center, 1991, p. 117).
Regardless of the differences, however, the invocation of the name would tie this new form of government to the sacred past, and make it easier to accept.
Mosiah as Lawgiver: John W. Welch has examined Mosiah in his role as a lawgiver:
“The law of Mosiah primarily made procedural changes and probably did not make radical changes in the substantive rules of the law of Moses. Mosiah instructed the new Nephite judges to judge “according to the laws … given you by our fathers” (Mosiah 29:25; italics added), and twenty-two years later the Nephites were still “strict in observing the ordinances of God, according to the law of Moses” (Alma 30”3) …
The law of Mosiah…prohibited slavery in the land of Zarahemla, for Ammon assured his converts that “it is against the law of our brethren, which was established by my father, that there should be any slaves among them” (Alma 27:9). Previously it had been only by royal benevolence that slavery was not allowed in Zarahemla (see Mosiah 2:13).
The law of Mosiah probably also provided that the governor alone had jurisdiction over capital offenses (see 3 Nephi 6:22), but this regulation may have been introduced a few generations later...
Mosiah’s judicial reform remained solid for sixty-two years, but then his laws were “altered and trampled under their feet” (Helaman 4:22). The majority of the people chose evil (see Helaman 5:2), Nephi had to deliver the judgment-seat to Cezoram (see Helaman 5:1), and judicial corruption soon ensued. (see Helaman 8:4; 3 Nephi 6:23).”Welch, John W. “The Law of Mosiah.” In: Reexploring the Book of Mormon. FARMS 1992, pp. 159-161. It should be noted that Welch and I disagree on certain aspects of Mosiah’s political reform).