The trick is coming up out of your front squat and giving the bar just enough momentum to regret it and then catch it on soft knees with a really tight core. Ideally you don't have to change your hand position too much, just getting a full grip on the barbell. Maybe just practice this in the front squat. Get a very relaxed grip, squat down, drive up, and try to regrip. A little practice should get this skill feeling routine. Like anything, practice makes perfect!