Licenses lie at the heart of open source -- and many other kinds of "open" too. That's because they are used to define the rights of users, and to ensure those rights are passed on -- that the intellectual commons is not enclosed. Their central importance explains in part the flamewars that erupt periodically over which license is "best" -- many people have very strong feelings on the subject.
Those heated discussions are one reason why it's rare for an open source project to change its license -- usually it's just easier to stick with what you've got rather than provoke another argument over which new license should be adopted. But there's another major impediment to changing the licensing: the need to get absolutely everyone who has contributed code to agree formally.
That's not a problem when the code has been assigned by contributors to a single entity, often a software foundation, as happens with components of Richard Stallman's GNU (GNU's Not Unix) project.
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