Wouldn't this change violate the terms of the previous copy-left license?
It is indeed true that the GPL itself does not give permission to offer another's work under more permissive terms like the MIT/X11 license.
Of course, nothing precludes a copyright holder from allowing their work to be made available under other terms besides the GPL. The relevant question is: does there exist any person who contributed their copyrightable work into the project under the GPL (and no other terms) and now objects to the distribution of their code under more permissive MIT/X11 terms? If so, that person could sue (or credibly threaten to sue) based on infringement of their copyright due to distribution under unauthorized terms.
This case would not be true if all contributors offered their contributions under more permissive terms, e.g., according to a contributor licensing agreement (CLA) that allowed the project owner to distribute their work under more permissive terms.
In the United States, there is an interesting legal concept of a "joint work" wherein all contributors of the work have full independent standing to distribute and offer licenses to the entire work. However, I am not sufficiently well versed in law to say when circumstances are sufficient to create joint ownership in a particular work.