USDF FEI Level Trainers Conference, Day 2, 2008. Great discussion on training flying changes. If you need good info on when or when not to train changes this is a super tutorial on the subject. Jan Brink goes through the half steps and canter pirouettes in a thorough and methodical method of teaching them.
Theory and discussion on how to begin training flying changes. Jan trains changes as soon as the horse has good balance, with no regard to age. The sooner the better because it relieves pressure of having to get it done at a certain time. The most important thing is the the horse has good balance in the canter and is straight before the change.