Changing tiers is always okay, although it may take as much as 30 days for the change to take effect. You are responsible for managing your membership through My Account at taijifit.com (top right of the page).
You can change at any time unless you have used your Membership discount to purchase a class or enroll in a class for free. If you have used your Membership discount to purchase a class or enroll in a class for free, you are required to maintain your Membership for a period of no less than four months from the purchase/start date of the class.