If you bought your license before the price changes, then it is truly a one time payment. If you bought your license after the price changes, then you will have to renew annually.
A sure way to find out (without bothering the developers), is to login to your account, and head on to the Subscriptions tab. If there are no subscriptions, your license is for a one time fee, and you will receive updates forever.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users