No idea how they did it; nobody ever sued us over it. We did continue to distribute some source to our clients, but not GPL. It may have been a kind of LGPL or something, and then V3 and V4 of the product simply imported the V2 as a library to build on top of

> Updating GPL code and making the resulting product proprietary is one of the main things the GPL is supposed to prevent.

It works perfectly fine if you own the copyright to all your code. You can't relicense somebody else's code of course, but you can relicense your own. So a company that develops something in-house and then slaps the GPL on the public release can change their mind on the license for future releases all they like.

It only gets tricky if they accepted external contributions. In which case there's often a contributor agreement, in which the contributor agree the company can relicense the contributed code.

You are correct, of course. I was writing in the context of the above story, but I failed to clearly express that.

I should have written,

"Updating someone else's GPL code ..."

Thank you.