Signal for flying lead change

If you are riding in a circle or arena. the best way i can describe it is to tilt the nose slightly to the outside and tap just behind the girth with the inside leg to tell that hind leg to reach further under him.
that way the inside front leg will become the leading let at the canter. 
after a while the horse will pick up the canter with just a tilt and feel of the bit move in his corners of his mouth on the outside . i am sure you remember, and perhaps just practice  
you should. after a while pick up the canter with the signals from a standstill. and yes the horse will understand if you gave him the signals at a trot. but not more than a couple of trot steps .  
if he does not go into the canter  bring him down. walk and try again. 
 

I SHOULD READ THE QUESTION .  sorry late and tired. yes it is different. just a tap with your heel and he should change . so to go from left to right signal with the foot to the right. 
you can canter on a circle and than ask for a change of direction also and he should change . as you change direction and give alternate aides. 

I just  pick up the rein slightly in the direction i want to go in. and that lifts the shoulder.  riding with two hands cause i ride english i use my ;pinkie to draw up the rein and horse feels it in his mouth and the change. is easy .