Possibly true... but wehther it can be done without modification to the code is a separate issue from whether a license change can be made without the consent of all contributors.
What I am saying is that anyone can take MIT licensed code and re-release it as GPL licensed code. That includes anyone of the author without consent from the other authors, or even someone who was not an original author. Specifically because such a change does not violate the terms of the MIT license. Whereas for going the other direction; GPL -> MIT you need consent from all authors (either directly or via a CLA that permits such a change of license).