To put this in our current situation, it would be like me saying that I have been intending to preach on marriage, but I feel it is more important at this time for me to start a series on dealing with false doctrine due to its current influence in our culture. The reason for the change is not a last-minute anointing of the Holy Spirit, but because of the expediency of the subject for the current situation. It says nothing about preparation and study. It is assumed that Jude is prepared to speak to the issue of his conviction precisely because of the presence of his conviction.
In the end, we need to be careful. From conception, preparation, to presentation, we can only hope that God is guiding it all. Can God change our sermon or lesson while we are in the pulpit? Of course. The question you should ask is whether or not this is a model that we should expect. Your message can be further shaped, nuanced, and impassioned while you are teaching, but this is not really God changing your sermon.
Preach what you prepare for and prepare for what you preach.