To a certain extent I’d say that you’re probably asking somewhat a bit too early, and I think you have a bit of a misnomer in there.
In terms of the misnomer, I’m sure lots of people have upgraded to the Version 3 of the subscription functionality of collecting email addresses and using the platform itself to send out emails via Mailgun. Following that, I think you then mean to ask “Has anyone moved from an ad revenue model to a membership service?”, seeing how subscriptions is the email collecting functionality while members is the revenue functionality.
Anyway, much like how memberships and subscriptions is currently in beta and somewhat limited, I take it that there are some people/organisations out there who by extension can’t move out of beta themselves due to certain limitations imposed by the nascent functionality (that’s not a diss on Ghost in the slightest). Exceptions aside, some of those people/organisations may still even be in alpha, currently in the process of building out in order to get to somewhat of a beta phase, much like the functionality itself.
Give it a month or two and perhaps some of those people/organisations will be emerging from alpha into beta.