On an ideal case you should make these changes with enough time. Remember that a change of plan isn't effective until the next renewal, so if what you wanted is to enjoy your new plan from the moment of your renewal, we can solve it for you.
You only have to speak to your therapist and he/she will contact us to solve it. Perhaps the change implies that your therapist requests a refund in case the plan you want to change to is of a lesser cost than your current one, or an extra payment if its the other way round.. In any case, do not worry, your therapist will guide you step by step.