> But only if you redistribute it. Otherwise you are quite free to keep
> any changes you make close to your chest*
That's a somewhat moot point; you only need to distribute source for any program you might distribute (N.B.: all the source, not just changes). Nevertheless, the code is still covered by GPL if you don't redistribute - it just doesn't mean you have to do anything to comply.
> So purchasing the commercial licence gets you what exactly?
The right to redistribute without complying with the GPL.
> Or are you saying that if you purchase a commercial licence you can
> modify the code and redistribute it as a closed source product?
> I don't see how that can work.
You don't need to.
But if you were a closed-source vendor that wanted database functionality in your product, you might have bought a commercial licence from MySQL. They have customers...