Can the Annual billing cycle be availed of by an existing iWantTFC customer? Is it limited to new customers only?

Yes you may. Any Paid Subscription Plan whether on a monthly or an annual billing cycle may be purchased by customers who are currently on, or have yet to purchase, a Paid Subscription Plan, in all countries/territories, except in the countries/territories sanctioned by the US Embargo and Sanction Laws. Subscribe at https://iwanttfc.com and go to the Account page.