Taking over another sermon and preaching it as if it were yours is always and unequivocally wrong, and if you do it you should resign or be fired immediately.
The wickedness is along at least three axes:
(1) You are stealing.
(2) You are deceiving the people to whom you are preaching.
(3) Perhaps worst, you are not devoting yourself to the study of the Bible to the end that God's truth captures you, molds you, makes you a man of God and equips you to speak for him.
If preaching is God's truth through human personality (so Phillips Brooks), then serving as nothing more than a kind of organic recording device in playback mode does not qualify. Incidentally, changing a few words here and there in someone else's work does not let you off the hook; re-telling personal experiences as if they were yours when they were not makes the offense all the uglier. That this offense is easy to commit because of the availability of source material in the digital age does not lessen its wickedness, any more than the ready availability of porn in the digital age does not turn pornography into a virtue.
In another place Carson wrote:
The bad way to listen to the sermons of others is to select one such sermon on the topic or passage you have chosen and then simply steal it, passing it off as if it is your own work. This is, quite frankly, theft, and thieves, Paul tells us, will not inherit the kingdom of God (1 Cor 6:10).
Yet in some ways that is not the most serious aspect of this form of plagiarism. Rather, it is the deep damage you are doing to yourself and others by not studying the Bible for yourself. Ministers of the gospel are supported by their congregations so they will give themselves to the ministry of the Word and prayer. That demands rigorous study. A faithful minister of the gospel is never merely a biological tape recorder or CD, thoughtlessly parroting what someone else learned, thought through, prayed over, and recorded. Indulge in this exercise and before long you will starve your own soul---and, no matter how good the sermons you steal, your ministry will sooner or later, and deservedly, become sterile, for the stamp of inauthenticity will be all over you.
R.R. Reno once explained why plagiarism is wrong in higher education:
When I assign a paper, I'm asking my students to analyze material and tell me what they think. Plagiarism amounts to avoiding the assignment, and turning in something that appears to be one's own analysis. The transgression---dishonesty---is not complicated, and it has nothing to do with theories about the possibility or impossibility of originality.
Plagiarism is a problem in higher education, because it involves students (and professors, who also plagiarize) who lie. They lie about what they know. They lie about what they have considered and thought about. They lie about their competence. Their success contributes to creating a culture of impostors.
If this is true in the classroom, how much more so behind the pulpit in the house of the Lord?