在開源軟體開發中，軟體再許可指在軟體模塊的軟體許可證不兼容並且需要兼容以進行更大的組合工作時的改變許可證操作。以二進制形式存在的，受版權保護的作品應用於軟體原始碼的許可證[1]可以包含矛盾的條款。這些要求可能導致無法將多個軟體作品的原始碼或內容組合起來創建一個新的組合作品。[2][3]
有時開源軟體項目會陷入許可證不兼容的情況。通常，解決這種情況的唯一可行方法是重新許可所有參與的軟體部分。為了成功地重新許可，需要得到所有相關版權所有者（通常是開發人員）的許可。雖然在免費和開源領域中，由於涉及許多貢獻者，實現所有作者的 100% 覆蓋通常是不可能的，但通常假設絕大多數就足夠了。例如，Mozilla假設 95% 的作者覆蓋率就足夠了。[4]自由和開源軟體領域的其他人，例如Eric S. Raymond ，就整個代碼庫的重新許可要求得出了不同的結論。 [5]
Mozilla項目和他們的Firefox瀏覽器是出於許可證兼容性原因成功重新授權的開源項目的早期示例。 Netscape的Communicator 4.0瀏覽器的原始碼最初於 1998 年在Netscape Public License / Mozilla Public License[6]下發布，但被FSF和OSI批評為不兼容。 [7] [8] 2001 年左右，時代華納根據 Netscape 公共許可證行使其權利，並應 Mozilla 基金會的要求，將 Mozilla 中所有在 Netscape 公共許可證下的代碼（包括其他貢獻者的代碼）重新許可[9]為 MPL 1.1/ GPL 2.0/ LGPL 2.1 tri-license ，從而實現 GPL 兼容性。 [10]
Vorbis庫最初被許可為 LGPL，但在 2001 年，在Richard Stallman的支持下，該許可被更改為BSD 許可，以鼓勵他人採用該庫。[11][12]
由於許可證兼容性，VLC項目還有一段複雜的許可證歷史：2007 年，出於許可證兼容性的原因，它決定不將許可證升級到剛剛發布的GPLv3。[13]在 2011 年初 VLC 從Apple App Store下架後，2011 年 10 月，VLC 項目將 VLC 庫部分從 GPLv2 重新授權到 LGPLv2，以實現更好的兼容性。[14][15]2013 年 7 月，VLC 應用程式可以重新提交到根據 Mozilla 公共許可證重新授權的iOS App Store。[16]
7-Zip的LZMA SDK，最初在GNU LGPL和Common Public License下獲得雙重許可。[17]連結二進制文件例外，由Igor Pavlov於 2008 年 12 月 2 日置於公共領域。 [18]
GNU TLS項目在2011年採用了 LGPLv3 許可證，但由於嚴重的許可證兼容性問題，在2013年將其代碼重新授權回 LGPLv2.1。[19] [20] [21]
GNU 自由文檔許可證1.2版與廣泛使用的創用CC署名-相同方式共享許可證不兼容，例如，對於維基百科來說，這是一個問題。[22]因此，應維基媒體基金會的要求，FSF 在 GNU 自由文檔許可證1.3版中添加了一個限時部分，允許使用 GFDL 的特定類型的網站在 CC BY-SA 許可下額外提供其工作。[23]繼 2009 年 6 月之後，除了先前使用的GFDL之外，維基媒體基金會通過雙重許可將其項目（例如維基百科）遷移到創用CC署名-相同方式共享作為主要許可，[24]改進了與更大的免費內容生態系統的許可兼容性。[25][26]
2010 年，爲了簡化許可證文本，OGRE項目將其許可證從 LGPL 更改為MIT 許可證。[27] [28] [29]
另一個案例是Google為他們的Android庫Bionic將 GPLv2 許可的linux 內核頭文件重新許可為 BSD 許可。為了擺脫 GPL，谷歌聲稱頭文件已從任何具有版權的作品中清除，將它們減少為不可版權的「事實」。[30] [31]但是，休斯頓大學法律中心的法學教授 Raymond Nimmer 對這種解釋提出了質疑。 [32]
POV-Ray自 1991 年以來，根據 FOSS 不兼容、非商業來源可用的自定義POV-Ray 許可證分發。[33][34]2013 年 11 月，其在Affero 通用公共許可證第3版（或更高版本）下重新獲得許可。[35]POV-Ray 是在 FOSS 許可證被廣泛使用之前開發的，因此開發人員編寫了自己的許可證，後來由於許可證與 FOSS 生態系統不兼容而成為問題。
2014 年，由於 GPLv3/GPLv2 出現兼容性問題， FreeCAD項目將其許可證從 GPL 更改為 LGPLv2。[36] [37]
2014 年，Gang Garrison 2從 GPLv3 重新授權到MPL ，以提高庫兼容性。[38] [39]
此外，Dolphin項目在2015年5月將其許可證從「僅 GPLv2」更改為「GPLv2 或更高版本」，以實現更好的兼容性。[40]
2015 年 6 月，mpv開始了項目的 GPL 許可原始碼的重新許可過程，以提高 LGPLv2 下的許可兼容性，並獲得了大多數（95%+）的貢獻開發者的同意。[41]2016年8月大約可以聯繫到 90% 的作者並表示同意。2017年10月，轉換完成。 [42]
2015 年 7 月，為改進許可證兼容性，尤其是與Git的兼容性，Seafile從 GPLv3 切換到 GPLv2。[43] [44]
2015 年，Natron從 MPL 重新授權到GPLv2 ，以實現更好的商業化。[45]
2016 年，MAME在為自己的書面自定義許可和非商業許可條款苦苦掙扎多年後，實現了將代碼庫重新許可到 BSD/GPL[46] 。 [47] [48] [49] [50]
2016 年 8 月，MariaDB公司將資料庫代理伺服器 MaxScale 從 GPL 重新授權給非 FOSS 但源可用且有時間限制的商業源許可證（英語：Business source license）[51]，三年後默認返回 GPL。[52][53]2017年發布了 1.1 版，並根據Bruce Perens的反饋進行了修訂。[54][55]
很長一段時間以來，D後端原始碼都是可用的，但並非以開源許可證授權。[56]因為它是在賽門鐵克部分開發的，不能以開源許可證重新許可。[57]2017年4月9日，後端部分也可以重新授權給開源的Boost Software License。[58][59][60]
自2004年該遊戲開放以來，微軟研究院的太空作戰模擬器Allegiance的許可證爲MSR共享源許可證[61][62]。2017 年 7 月 27 日，更改為 MIT 許可證。[63][64]
O』Riordan, Ciaran. (About GPLv3) Can the Linux Kernel Relicense?. fsfe.org. 2006-10-06 [2015-05-28]. （原始內容存檔於2011-07-21）. Someone who works with many lawyers on free software copyright issues later told me that it is not necessary to get permission from 100% of the copyright holders. It would suffice if there was permission from the copyright holders of 95% of the source code and no objections from the holders of the other 5%. This, I’m told, is how Mozilla was able to relicense to the GPL in 2003 despite years of community contributions.
Licensing HOWTO （頁面存檔備份，存於網際網路檔案館） by Eric Steven Raymond&Catherine Olanich Raymond "Changing an existing license [...]You can change the license on a piece of code under any of the following conditions: If you are the sole copyright holder[...]If you are the sole registered copyright holder[...] If you obtain the consent of all other copyright holders[...]If no other copyright holder could be harmed by the change" (accessed on 2015-11-21) Mozilla Relicensing FAQ Version 1.1. mozilla.org. （原始內容存檔於2010-05-13）. Some time ago mozilla.org announced its intent to seek relicensing of Mozilla code under a new licensing scheme that would address perceived incompatibilities of the Mozilla Public License (MPL) with the GNU General Public License (GPL) and GNU Lesser General Public License (LGPL).
February 2001 （頁面存檔備份，存於網際網路檔案館） on xiph.org "With the Beta 4 release, the Ogg Vorbis libraries have moved to the BSD license. The change from LGPL to BSD was made to enable the use of Ogg Vorbis in all forms of software and hardware. Jack Moffitt says, "We are changing the license in response to feedback from many parties. It has become clear to us that adoption of Ogg Vorbis will be accelerated even further by the use of a less restrictive license that is friendlier toward proprietary software and hardware systems. We want everyone to be able to use Ogg Vorbis."" Denis-Courmont, Rémi. VLC media player to remain under GNU GPL version 2. videolan.org. [2015-11-21]. （原始內容存檔於2015-11-22）. In 2001, VLC was released under the OSI-approved GNU General Public version 2, with the commonly-offered option to use "any later version" thereof (though there was not any such later version at the time). Following the release by the Free Software Foundation (FSF) of the new version 3 of its GNU General Public License (GPL) on the 29th of June 2007, contributors to the VLC media player, and other software projects hosted at videolan.org, debated the possibility of updating the licensing terms for future version of the VLC media player and other hosted projects, to version 3 of the GPL. [...] There is strong concern that these new additional requirements might not match the industrial and economic reality of our time, especially in the market of consumer electronics. It is our belief that changing our licensing terms to GPL version 3 would currently not be in the best interest of our community as a whole. Consequently, we plan to keep distributing future versions of VLC media player under the terms of the GPL version 2. [...]we will continue to distribute the VLC media player source code under GPL "version 2 or any later version" until further notice.
Mavrogiannopoulos, Nikos. The perils of LGPLv3. gnutls.org. 2013-03-26 [2015-11-18]. （原始內容存檔於2016-04-02）. LGPLv3 is the latest version of the GNU Lesser General Public License. It follows the successful LGPLv2.1 license, and was released by Free Software Foundation as a counterpart to its GNU General Public License version 3. The goal of the GNU Lesser General Public Licenses is to provide software that can be used by both proprietary and free software. This goal has been successfully handled so far by LGPLv2.1, and there is a multitude of libraries using that license. Now we have LGPLv3 as the latest, and the question is how successful is LGPLv3 on this goal? In my opinion, very little. If we assume that its primary goal is to be used by free software, then it blatantly fails that.
Licensing update rolled out in all Wikimedia wikis （頁面存檔備份，存於網際網路檔案館） on wikimedia.org by Erik Moeller on June 30th, 2009 "Perhaps the most significant reason to choose CC-BY-SA as our primary content license was to be compatible with many of the other admirable endeavors out there to share and develop free knowledge" Cason, Chris. Download POV-Ray 3.7.0. 8 November 2013 [11 November 2013]. （原始內容存檔於2022-03-25）. Starting with version 3.7, POV-Ray is released under the AGPL3 (or later) license and thus is Free Software according to the FSF definition. […] Full source code is available, allowing users to build their own versions and for developers to incorporate portions or all of the POV-Ray source into their own software provided it is distributed under a compatible license (for example, the AGPL3 or – at their option – any later version).
Prokoudine, Alexandre. LibreDWG drama: the end or the new beginning?. libregraphicsworld.org. 2012-12-27 [2013-08-23]. （原始內容存檔於2016-11-09）. [...]the unfortunate situation with support for DWG files in free CAD software via LibreDWG. We feel, by now it ought to be closed. We have the final answer from FSF. [...] "We are not going to change the license."
license. freecadweb.org. 2014 [2015-03-25]. （原始內容存檔於2016-12-04）. Licences used in FreeCAD - FreeCAD uses two different licenses, one for the application itself, and one for the documentation: Lesser General Public Licence, version 2 or superior (LGPL2+) […] Open Publication Licence
Planned license change (GPL -> MPL), Help needed. Gang Garrison 2 Forums. 2014-08-23 [2015-03-23]. （原始內容存檔於2018-06-12）. tl;dr: The current license prevents us from using certain nice and (cost-)free libraries / frameworks, so we want to change it. The new license (MPL) would be strictly more free than the old one, and is the same one that's also used by Firefox.
bsl （頁面存檔備份，存於網際網路檔案館） "Change Date: 2019-01-01, Change License: Version 2 or later of the GNU General Public License as published by the Free Software Foundation." on mariadb.com (August 2016)
