There is quite a difference in the way flying changes are viewed in jumping and dressage worlds. In jumping they are necessary utility movement, taught to horses early in training with sometimes exaggerated flexion and being a bit late behind is not considered a big deal. Also, many jumping horses cannot counter canter.
In dressage flying change appears only in Third level. Second level has counter canter and collection. This makes flying change quite an advanced movement. Even though talented young horses can learn flying changes early - for dressage they must be straight, on the aids and clean. Accidental change once in a while does not mean the horse understands the aids for it. Aids need to be taught and the horse must wait for the rider to ask instead of anticipating it.

Traditionally for dressage, it was taught that horses must first learn counter canter, straight walk-canter-walk transitions, strike into any lead any direction and hold shoulder-fore in canter then they are ready for the work on flying changes. This approach is still very much true for horses who do not have super balanced, elastic canter with good jump. Modern warmblood breeding is creating horses who have much better canter. These horses can learn flying changes earlier due to better mechanics of their bodies. An exceptional horse like Valegro was able to do sequential changes at the age of 5.

Either way, one important component - a rider must also learn how to do flying changes, This includes several aspects:

• Rider's seat must be supple enough to match horse's back during canter, equally well on both leads. Any unsteadiness with the hands, clamping with legs, thighs, stiff joints, pushing into stirrups, excessive crookedness will make a flying change much harder endeavor for the horse and the rider alike
• Rider must be able to ask for walk-canter transition on a straight line on either lead without exaggerating canter aids, positioning of the horse or his/her own body
• Rider must feel which lead he/she is on without looking down
• Rider must be able to ride very straight canter on either lead with even elastic contact. Excessive bending in the neck or body is counterproductive for changes
• Asking for the change happens during stance phase of the canter stride. Rider must be able to feel the canter beats and suspension phase, feel and support rhythm and tempo of the canter
• Aids for the change are lateral: same side leg and rein, leg swings back, hand closes for a half-halt without moving backwards. For example, on the right lead right hand closes and right leg swings back, they are new outside aids. New inside leg moves forward to the girth position. It all happens simultaneously during stance phase of the canter stride. Next moment the horse jumps off the ground and performs flying change to the left.
• Disturbances to the flow of canter initiated by the rider are pushing with the seat, pulling on the reins, stiffening, stopping rider's seat motion, losing balance during the change, twisting out of the saddle, leaning to one side, etc
• The change is forward movement. Try skipping and changing leg occasionally. Notice how you jump forward not sideways, notice how rhythmical and forward the motion is, no stopping, pausing, or tensing necessary

It takes time and practice to develop enough feel and coordination to ride balanced, smooth changes on a horse who knows how to change. Learning on a school master is extremely helpful, allows faster learning, less frustration for both horse and rider and a great confidence builder.

Happy riding...