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I came across bootstrapvalidator project on Github which was released under MIT license in the past. In the contributors list, I can see a lot many people other than the main author of the project. The latest releases of the project are under a commercial license.
The MIT license is considered a fairly permissive license for a number of reasons, including the sublicense clause:
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Emphasis mine
It's this portion of the MIT license that allows anyone to take an MIT license and re-license so long as they maintain the requirements stated with The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
Is it legal for the author to change the license from MIT license to a commercial one, if he is the only contributor?
Yes, as the sole author of the project, they own the copyright to the project. Owning the copyright allows them to re-license the project to a commercial license.
In this particular case, the copyright owner to the project can re-license as they wish and does not need to maintain the copyright or permission notices from the MIT license.
Is it legal to change the license from MIT license to a commercial one, if there are other contributors involved?
The project could be re-licensed in any of the following cases:
In the first two cases, the copyright owner(s) can re-license without maintaining the copyright and permission notices.
In the third case, the person re-licensing the project must obey the terms of the license and they must maintain the copyright and permission notices.
Also note that the MIT license is not a copyleft license, so there is no obligation to share the associated source code.
Likewise, just because someone chooses to fork and re-license a project, that re-licensing applies only to the forked project. The re-licensing has no bearing upon the original project or how the original project is licensed.
The author can change the license going forward. Proponents of the license consider this a major selling point. The sole author of a project can change the license at any time. Others can include it in their own products, and change the license at any time.
The author can continue to use other people's MIT-licensed code in the new, non-MIT-licensed version. The other people's code is still MIT-licensed, but as long as the author abides by the licenses for these other people's code, there is no problem.
Or the author can ask the contributors to relicense their code too. There was a point when Mozilla wanted to change the license to its code, and it had to go around asking the contributors for permission. If everyone agrees, good; you can relicense. If someone says no, then you've got a problem.
What the author can't do is change the license going backward. If Version 2 of a project is MIT-licensed, and then the maintainer changes the license for Version 3, then that's the maintainer's prerogative: starting from Version 3, the new license applies. But Version 2 is still MIT-licensed, and people can still use and/or fork that version if they wish. That genie is out of the bottle.
