A very good question. To which I don’t have an easy solution.
I would write a custom function (and run it once, then delete it) that loops through all your users (via a WP_User query), and for each user, use the update_field function (documented on this site) to save a value of 1 for the field.
I hope this makes sense.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users