keep it as short as possible. Say a little about yourself, maybe include something personal like a hobby. Tell them how much you look forward to working with them and that you will take the time to get to know them all through a SHORT personal meeting. I would not announce plans for change even if you have any.
It’s much more important that your first interactions are positive.
Good luck in your new job.
Karin hit the high points.
Who you are.
Basics of your fitness background.
One fun personal fact.
Excited to meet them.
Opportunity / opportunities to meet them.
Depending on how many instructors you are supervising and how many locations, you might want to schedule a staff meeting or three meet and greet times (different days and times) where they can drop in and meet you.