What form would the transaction take?
I am low-key building a marketplace for this, because this really should exist. The problem, as a marketplace, is identifying projects both important enough that people will pay for it and aren't* already functionally owned by a company that has a vested interest in their own project management.
(Are you a maintainer who's interested in something like this? Are there projects you would pay for this for? I want to hear from you. com dot edropple at ed)
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.
It's a little much to ask from an unpaid open source maintainer, and if you really want it that bad you should be willing to pay for it. Companies like Red Hat have made a living off of people willing to make that tradeoff.
I mean, you could have separate bugfix and feature releases for a relatively low cost, but that would only alleviate the problem for the duration of a release cycle.