How do you start charging beta users who have enjoyed everything for free for over a year?

During our year-long private beta, we promised our first wave of users free access in exchange for their honest bug reports and feedback. That partnership was invaluable for shaping the product, but now that we are officially launching commercially, those same beta users expect their accounts to stay free forever. Introducing a paid billing wall to our day-one supporters feels like a betrayal of trust.
How do you successfully introduce pricing to a community that has known your product as completely free since its inception?