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If you alone hold the copyright to a piece of software you can change the license at any time.

But that doesn't apply to versions that already had an open license. You can't retroactivally revoke a license, you can only add it to new versions.

That's not true, and it's happened before:

http://www.cnet.com/news/nessus-security-tool-closes-its-sou...

A project like Linux couldn't do this, since it has thousands of authors, and each author would have to agree to the change. But if the author is a single person, or a corporation, then only one person has to make the decision.

You're misreading the article, I think. They didn't release a new version under the GPL. They didn't retroactively undo licensing for older versions.

It is not at all clear that you can snap your fingers and renegotiate an existing license arbitrarily like that. You need to do it in advance of the agreement.

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