Changes in license terms

Chuck Swiger chuck at
Thu Aug 12 23:07:07 UTC 2010

Hi, Michael--

On Aug 12, 2010, at 3:28 PM, Michael Bradley wrote:
> If a developer changes the license of some library he maintains (taking for granted he is the sole copyright holder), say going from the MIT/expat license to LGPL v3, what are the principles of law, say in the USA, which prevent the new license terms from retroactively affecting earlier versions of his code released under different terms.

Well, Mother Nature usually disapproves of time-travel. [1]

Once you release a version of something to the public under the terms of a particular license-- at least, for anything which the OSI lists as open source-- that version of the software can be redistributed to anyone else under the same terms.  This remains true regardless of whether the copyright holder chooses to release a new version of that software under different terms, or even if the original version of the software is re-released under different license terms.

The notion of estoppel comes in because someone performing redistribution of the original software had relied on an explicit promise made by the terms of the original license which gave them the permission to redistribute the software under that license.  Once someone takes action based upon such a promise, the copyright holder cannot retract it; you lose the right to assert that someone following the original terms is violating your copyright.  See:


[1]: At least for things which are not sub-atomic particles, but thanks very much anyway, R. Feynman.

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