You mark releases as "Stable and supported" and have corporations subscribe against releases. If you are subscribed to a release, you get bug fixes for that release. If someone does not have a subscription to a release, it does not get updated, and if there is no current subscription for that release, you don't accept new ones.

You always allow people to move forward their subscriptions, but never backwards. You provide migration paths for each supported release.

People will pay for it.