In reply to Sean Lang.

Well, you have your motives but it seems stupid to not have pushed your improvements into the original free plugin, https://github.com/bradt/wp-migrate-db although there is no guarantee they would have been accepted, at least the denial of improvements would have been sturdier ground to stand on for your actions.

As for closed software, the license validation inside the plugin does not make the plugin violate the GPL. The validation ensures you've made a purchase and thus, can access support and receive future upgrades.

Just out of curiosity, you know Brad is trying to make a living through WP DB Migrate Pro via Delicious Brains. By keeping the plugin named the same thing, you're hurting his business and confusing users at the same time. If it was a rewrite, then even more so it should have been renamed. By keeping it the same name, it looks like you're capitalizing on the plugins popularity, despite not attaching your own price tag to it.

I really wish I could do this without interfering with Delicious Brains, but I don’t have any good solution for that.

Easy, rename it, remove any mention of Brad with the exception of the copyright and make it your own. Then you can do with it as you wish.