This is how I was trained to change my lead over the fence and this has worked on every horse I've ever jumped, whether it was trained by me/my trainer or not. Over the jump shift your weight to the outside stirrup to balance your horse and open your inside rein to turn their head. So if you were to be on the right lead trying to switch left you would shift your weight to the right and open your rein left. As for knocking rails because of it, as long as you aren't shifting so dramatically your horse is losing balance or pulling their head so hard that they're turning midair or right on landing you should be fine. Good luck!
First of all, in my opinion you don't have to change your lead over jumps.
But you can change your lead shortly after your jump if you want. The problem if you change leads over jumps you risk to drop your jump and prevent your horse jumping correctly. Also over jumps you may watch where you want to go after to predict the curve.