Looking at the starting post, the instructor in question would probably trash most of you for talking the exact scientific bullsh!te
1st Newton's law: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Hence, to bring a craft into a turn from a straight flight requires action of force. The energy to generate that action of force must come from somewhere (energy is conserved), and that's from its kinetic energy of forward motion.
That being said, if you had a ball flying in a vacuum and wanted to change its direction of flight without touching it (external input=force added into system=unacceptable), you would have to slow the ball down. That exactly applies even to electrons orbiting around atomic core and any lift-drag and other bollox is totally irrelevant here.