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 will contact us to solve it. You might have to fill in a refund form 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 caes your therapist will help you through it.