They need a solid counter canter, very well established simple changes (canter-walk-canter with only 2 strides maximum of walk) and the true canter should be extremely well balanced with an uphill tendency.
I would add to that-
The horse needs to have a solid understanding and response to a half halt, the rider also needs to have excellent balance and understanding of the timing of the half halt.
A horse that is too much on the forehand will not be able to do a flying change, likewise, a horse that is too far off the forehand will not hold a change.
Because they don't have a rider on top to try and balance as well ;)
But, if you are happy to simply throw your weight to one side and unbalance the horse enough to force a change, then go right ahead. Sure, the change will be there but it will most likely be broken, unbalanced, and the horse will become nervous as it had to struggle to stay upright.