The First Level dressage tests call for lead change through the trot. Here's how to get a good score for that movement as well as deal the horse that likes to do a flying change instead of waiting for you.
When you do the change through the trot in a test, you can do 3-5 trot steps. Center your trot strides over X. That is, if you do 3 strides of trot, the 2nd one is on X. If you do 4 strides, do 2 strides before X and 2 after X. If you do 5 strides, the 3rd one is on X. I often see all of the trot steps either before or after X, and that's not what you want to do.
If your horse anticipates a flying change, here are some exercises you can do:
1. Canter, trot, then immediately trot onto a 10-meter circle that's in the direction of the new lead. Stay on the circle until you feel him relax and not anticipate cantering. When that happens, continue along the diagonal, and wait until just before the corner before you pick up the new lead.
2. Canter, trot, then leg-yield with a bend away from your new inside leg. This will get him thinking about something else and also set him up for a good canter when you're at the end of the diagonal.
3. Canter, trot, then step into shoulder-in for the rest of the diagonal. Keep his hind legs on the diagonal and bring his shoulders in the direction of the new lead. At the end of the diagonal, you can either straighten and stay in trot (to avoid anticipating canter) or straighten to shoulder-fore and get a nice, engaged (because of the shoulder-in/shoulder-fore) transition to canter.
I like the approaches above because you're not saying "No! Don't do a flying change!" You're saying, "Let's do something else like circle, leg yield or shoulder-in instead."