I have heard it said that angels don’t change their minds once they make a decision but, if that is true, why did some of them change their minds about following God to Lucifer? Where in Scripture or Catholic teaching does it tell us they can’t or don’t change their minds?
The fallen angels did not “change their minds” from God to Lucifer. Like men, all of the angels were given a test of obedience. Some chose God, others chose themselves (foremost among them Lucifer). Those that chose self over God fell. Because of their purely spiritual nature and their superior intelligence, the fallen angels could not repent (just as the unfallen angels cannot now fall). As the Catechism of the Catholic Church states, that choice for God or for self was irrevocable:
It is the irrevocable character of their choice, and not a defect in the infinite divine mercy, that makes the angels’ sin unforgivable. “There is no repentance for the angels after their fall, just as there is no repentance for men after death” (CCC 393).