Has anyone ever tried:

Annual plan only available after 6 months of membership?

While I understand that sounds counterintuitive, if not odd, it:

a) offers an incentive to stay.

b) with that goal line in mind, the offer of annual when getting there has special meaning. As opposed to a random "you've stayed. You must be happy. Wanna go annual?"

Again. I hear what you're saying, but in a nudge-y / Hooked sorta way it might work.