When an LDS prophet speaks, if what he says is different than canonized LDS scripture, do the words of the prophet take precedence?
Are you referring to anything specifically? If you can provide a specific instance, that might be more informative.
In short: the answers are: Nullify, no. Takes precedence, yes.
For example, polygamy is an eternal law. And although it is not practiced today, it has been practiced as noted in the Bible (Genesis 16:3; 25:1; 29:21-30; 30:3-4, 9), spoken of in the Book of Mormon: (Jacob 2:30), and practiced in the beginnings of the LDS church. However, the prophet in 1890, President Woodruff, wrote a Manifesto which lifted the commandment to practice plural marriage LDS.org topic: Polygamy. In that instance, the prophet didn't "nullify" the law, because those who had practiced it would still be sealed in heaven, however the practice on earth was stopped because the Lord commanded it.
In general, scriptures were written by prophets, who received the revelation from God, and the words of the living prophets are considered scripture: "In addition to [the] four books of scripture, the inspired words of our living prophets become scripture to us. Their words come to us through conferences, the Liahona or Ensign magazine, and instructions to local priesthood leaders. “We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God” (Articles of Faith 1:9)." LDS Gospel Principles
Revelation from the living prophets are spoken as if from God. (D&C 1:38) Also, God does not change. (1 Nephi 10:18). Therefore, the revelation and the scriptures are from God, and God does not change, though he may give some specific instruction to certain people at different times, it doesn't nullify scripture.
This pattern is shown when Christ declared that the law of Moses was fulfilled in Matthew 5:17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.
In an interesting address by a modern prophet (an apostle at the time) to the students at BYU in 1980, President Benson said, "Beware of those who would set up the dead prophets against the living prophets, for the living prophets always take precedence." Fourteen Fundamentals in Following the Prophet
First of all, the title doesn't really match the question body. I suppose what you mean is something along the lines "God commanded X earlier, now he wants you to do Z instead."?!
There are multiple mentions of revoking an earlier command in D&C, for example:
31 Who am I, saith the Lord, that have promised and have not fulfilled?
32 I command and men obey not; I revoke and they receive not the blessing.
33 Then they say in their hearts: This is not the work of the Lord, for his promises are not fulfilled. But wo unto such, for their reward lurketh beneath, and not from above.
So, revoking commands is something the Lord explicitly said he would be doing. Other answers have mentioned modern day "changes" to commandments. Here are two biblical examples that popped into my head, I figure there are many more, but lack the time to look for them:
- law of Moses no longer applies after Jesus' sacrifice (do i need reference here?)
- Israelites were to conquer Canaan right away, got scared and God told them to travel 40 years in the desert instead. Weren't happy with that, tried to conquer anyway - didn't work out. (Numbers 14)
In short, God can and does change commandments. He tells this the prophets. That does in no way "nullify" or "change" something in retrospect - you had a command earlier, you have a command now - of course the current command takes precedent, why do we even talk about this? ;-) Note also that there are lots of changes all the time, but most go under the radar because they are mostly policy changes. These however do also represent what the leaders of the church feel (as in, feel inspired by the Holy Ghost) is the right thing right now.
Examples of recent changes that probably went under the radar:
- reduction of missionary age requirement
- new global guideline that the sacrament meeting should be the first meeting
Sure these are not doctrinal changes, but they change policies that have been in place for a long time (and I'm sure, well thought out by earlier leaders, and inspired at that time).
I believe there are several issues being discussed that are not necessarily all the same. There should be a distinct difference noted in the policies and running of "The Church", and the revelatory process relating to doctrine.
The change in age of missionaries, while inspired, is really administrative in nature. Hypothetically the age of missionaries could at any time drop to 13 or rise to 65, and there would be no doctrinal inconsistency. This would be highly unlikely, but it could happen and then change back six months later. We can argue the wisdom in that, but it doesn't nullify or contradict doctrine.
A different example would be that of polygamy. When it was instituted, it was, in fact, a reintroduction of a biblical doctrine shown throughout the Old Testament. When the manifesto was given it did not nullify the doctrine, but rather forbade the practice. There are still areas where this doctrine is still active such as when a widower is remarried, it is not actively practiced by current members of the LDS faith in good standing.
Where this would come to the test would be a canonised doctrinal announcement that would contradict established doctrine. The Word of Wisdom that was later canonised, is a good example of new doctrine being introduced via a modern Prophet. The interpretation of this doctrine has varied and evolved, but has been established to specifically be a prohibition against alcohol, tea, coffee, and tobacco (among other details). It is not unthinkable that a modern prophet could add or subtract from that list substantively, and current inspiration would rule.
That doesn't mean that our understanding of an established doctrine or principle couldn't be altered with new information from a modern Prophet.
No, but from there one has to dig in to the deliberately strong word you framed it with: nullify.
When it comes to the Mormon health code for example, they don't drink alcohol. So wine is not used in the sacrament of the Lord's Supper. But the commandment of Jesus to use wine in rememberance is not nullified but put on hold. There will be a time where wine is restored to the ordinance so it is not a word game.
Mormons don't think of it as a nullification even though there is an unambiguous instruction from the Lord (and that is a pretty good authority) and yet they are not going to do what he said. But they remember that the difference is just temporary so it is not considered as changing the canonized scripture.
The Joseph Smith translation of the Bible is another good example. The old definition of translation is useful here. But there you have a modern prophet literally changing the Bible but it is seen as a re-transmission of the canon in an original form.
Current revelation can be more relevant
Though God himself is the same yesterday, today, and forever. (Hebrews 13:8, 1 Nephi 10:18), man and his circumstances do change. President Brigham Young was asked to speak on this topic.
Brother Brigham took the stand, and he took the Bible, and laid it down; he took the Book of Mormon, and laid it down; and he took the Book of Doctrine and Covenants, and laid it down before him, and he said: "There is the written word of God to us, concerning the work of God from the beginning of the world, almost, to our day. And now," said he, "when compared with the living oracles those books are nothing to me; those books do not convey the word of God direct to us now, as do the words of a Prophet or a man bearing the Holy Priesthood in our day and generation. I would rather have the living oracles than all the writing in the books."
This isn't meant to disparage the scriptures, but rather to use their already esteemed status to stress the even greater importance of current revelation.
Had it not been for a living oracle, Israel would not have been lead out of Egyptian captivity. There was nothing wrong with what God had said to his people before that time, but what he said then to Moses was necessarily tailored to the Israelites' current predicament.
Existing commandments can change
The coming of Jesus Christ ended the shedding of blood. Instead of animals, sacrifices to God were to be of a broken heart and a contrite spirit. (3 Nephi 9:19) Thus we see that God is able to change his own commandments.
That which is wrong under one circumstance, may be, and often is, right under another. God said thou shalt not kill,—at another time he said, thou shalt utterly destroy. This is the principle on which the government of heaven is conducted—by revelation adapted to the circumstances in which the children of the kingdom are placed. Whatever God requires is right, no matter what it is, although we may not see the reason thereof till long after the events transpire.
History of the Church, 5:135
Joseph compared it to the raising of children. In some circumstances, a parent may punish a child for eating an apple (if it were stolen), but allow it in other circumstances (if it had been given to them). That may seen inconsistent to a young child, but eventually they will learn and understand.
An example in the latter days is the adoption and later un-adoption of polygamy. Another example is the command for the early Saints to collect themselves in one location (Zion), versus the instruction today to strengthen the Church in each nation they are in.
Eternal principals are of course eternal. The specific actions they necessitate will depend on time and circumstance, and these instructions are delivered by a living oracle.