to me, there are two sides to this story, and it depends a lot of what you call ‘new fitness concepts’.
Let’s use Pilates exercises as an example which have remained largely unchanged even though new science has demonstrated how some of the benefits have come about. To my knowledge, the Pilates schools have continued the development of teaching, though. So if you were to be a Pilates instructor, I would expect you to be aware of this discussion in this field but I would not consider it a risk if you are not if you are providing safe and effective exercise programs to your clients.
There is also the question of being out of date (meaning: not knowing of) versus being aware of and deciding not to use it. Just because something is a new fitness concept does not necessarily make it a better concept. And whether or not you should instruct it depends a lot on your level with familiarity with it and thus your ability to instruct it with safety and effectiveness in mind.
I believe the risks are greater for an instructor to latch onto the latest trend without really knowing it and being cautious as to when, where and for whom to apply.