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Relicensing versus license compatibility
Quoting: Relicensing versus license compatibility —
Some copyright holders, however, do permit recipients to further distribute the copy in their possession under a different license. For example, the copyright holder can release a work under a GNU GPL and add the option for recipients to follow a later version of the GNU GPL; however, this is optional and at the discretion of the copyright holder. They may also license the work without adding the option for others to upgrade to a newer version of the license.
If a copyright holder licenses the work under "GNU GPL version 2 or any later version," it means the work is effectively licensed under either the GNU GPLv2 or the GNU GPLv3 (or any later version that the FSF releases). Recipients of such a work can choose to further distribute their copy under the GNU GPLv3 instead of the GNU GPLv2, taking advantage of an existing licensing option explicitly granted to them by the copyright holder.
Note the subtle difference here: true relicensing can only be done by the copyright holder, since only they can freely change the license of the work to any other license. Relicensing explicitly permitted by copyright holder(s), in contrast, is when a non-copyright holder exercises the right given to them by the copyright holder(s) to further distribute the work under a different license. The non-copyright holder is limited to the permitted changes in licensing.