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I commited some work on GitHub under the MIT license.
Is it possible to move from MIT license to BSD-3 or 4 clauses for some of the projects? And as an extent, to any other license?
If possible, is there a particular thing to do other than changing LICENSE and upgrading files headers?
Same question could apply with GPL. Can I (somewhat say) 'downgrade' some work to BSD for example?
To be more concise, I use the terms upgrade/downgrade in terms of copyleft.
The question is more complex than some other analyses would have it.
When you receive a work created by some other person(s), A, and you make changes, B, leading to C (which is a derivative work of both A and B), there are a number of licences to consider.
The licence under which you received A continues to apply to A, when distributed simply as A; you have no power to vary that.
You may licence B under whatever terms you please, provided that they don't cause C to be under a licence which conflicts with the licence terms on A as regard derivative works.
C, being a work in its own right, is also covered by a licence. The choice of this licence may be constrained by the licences on A or B, or it may not be. Unless so constrained to be, it is possible that C's licence will not be the same as either that on A or B.
So, if A is covered by the X11 license (the FSF note that the term "the MIT license" is ambiguous, as there have been many), you may release B under the X11 license, and C will then very likely be under the X11 licence, unless you arrange otherwise. You may release B under the GPL, in which case C will also need to be under the GPL (per GPL2 s2b or GPL3 s5c, as appropriate). You may release B under either 3- or 4-clause BSD, in which case you will need to clarify what licence C is released under.
If A is covered by the GPL, you may release B under the same version of the GPL, or under a less-restrictive licence compatible with the GPL in question (which includes 3-clause BSD, but not 4-clause BSD). C will need to be under the GPL (see previous para).
Note that this analysis is currently thought to be persuasive in England and Wales (ie, I have taken legal advice that the line of analysis is likely to find favour with a court). I have no idea whatsoever if it would find favour in other jurisdictions, but you don't say where you're based. IANAL/IANYL, of course.
If it is your own work your can relicense it however you like. The only thing you cannot do is to renege on the original license with respect to copies that have already been conveyed to others.
