Youve put it brilliantly there flying changes would be "preferable" but is NOT necessary. Mainly they need to be responsive to you in the sense say they land after the jump and are on the 'wrong' leg for the way you need to go, they need to be able to correct it themself or be ready to instantly come back to trot and then correct it when you ask.
As said complete balance in flatwork is most essential first eg always impulsive, always go into the correct leg, are responsive to you.
Ive found practise makes perfect and you will always get horses (like my pony) who thinks after a few jumping rounds they "know" it all lol so you just need to make sure they are listening to you, able to lengthen/shorten strides incase you need to adjust for take off and slowly you can get them to learn by looking one way- or i found for one of mine opening the rein of the way you want to go after landing can encourage them to land on the correct leg for you to go around the course.
