No, once you have an MIT-licensed codebase without a copyright assignment scheme, you no longer have the freedom to relicense it at will. You could attempt to have a mixed-license codebase, which is supported by the GPL, and specify that all new contributions must accept the GPL, but this is tantamount to an incompatible fork of the project from the perspective of any downstream users, and anyone who insists on contributing code under the GPL has the freedom to perform this fork themselves.
This is simply false. You can accept GPL contributions and clearly indicate the names of the contributors as required by MIT. There is no "incompatible fork".
No, they openly refuse to accept any GPL code. And even have a strict policy of not even reading GPL code.
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