Yes, you can upgrade or downgrade your plan at any time, and can make those changes yourself right from your ABC Trainerize account.
Note that not all features are available or included with all plans, so be sure to review what is or isn't included in your new plan before completing the change. You will lose access to any features or add-ons not included in or available with your new plan as of your next billing cycle. For example, if you choose to downgrade from a Studio plan to a Pro or Grow plan, you may lose access to your included add-ons (Custom Branded App, Video Coaching, Advanced Nutrition Coaching, Stripe Integrated Payments).