Changes in license terms

[Michael Bradley]

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.

I've been googling and asking in places like #cc and #gnu on Freenode, but have yet to find a reference that explains this matter clearly.  My understanding so far is that this question is related to the concept of estoppel.

Thank you for any insights or references you can provide.

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Michael Bradley
www.holynamesoftware.org