In reply to Carl Hancock.

Agreed. This is why I opted not to sell a successful free plugin years ago. While negotiating a price, I got the feeling the buyer's main interest wasn't the plugin or its functionality.

A major part of what you're selling is the ability to push updates to an established user base.

I'd probably be in favor of a kind of "official" fork system, where users are alerted to a new version (published under a different slug), and WP offers a way to upgrade to the new plugin, if they trust the new fork.