Not a lawyer either but I believe if you hold the copyright for the entirety of the code you can do a release of any version with whatever license you want, if you don't hold the copyright to the entire code you can only do so with the consent of all copyright holders or by removing/re-implementing all code copyrighted by others (this would be too complex for projects like FreeCAD with dozens of contributors).
Isn't that exactly what copyright is about? Holding the copyright to something doesn't mean you are the author or that you claim to be. At least that's how I interpret it but this is probably different depending on language and jurisdiction.But there is something other more important, respect for the rights of others. Even if you have MIT license you need to attach the MIT license to the software and you have to clearly inform who is the author. You can't say I am the author, if you are not. You can but license for brand, for distribution, for profit %, but you can't buy being author. But yes, the author can sell all rights to his product, he will be author but he can't do anything with such product.
We're kinda off topic now though.