I'd like to relicense one of my projects from Apache 2.0 to Apache-or-MIT dual license for consistency with the outer community (Rust), and to allow GPLv2 compatibility, which I was previously unaware was disallowed by Apache.

From what I understand, it is necessary to get approval from all contributors to migrate from MIT to Apache, as that implies contributions are providing the Apache 2.0 patent grant. But what about the inverse? Since MIT is mostly a subset of Apache, and the dual MIT license is there just to serve as an outbound license that allows GPLv2 compatibility, does the conversion from Apache to MIT still require approval from all contributors?

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  • MIT to Apache doesn't require approval: by complying with Apache you're already complying with the MIT license, provided that you add the MIT notice to the NOTICE file. However, complying with MIT doesn't imply Apache compliance, e.g. MIT lacks requirements to display NOTICE files. Therefore, you need approval. The patent clause is a red herring as it only applies to contributions to an already Apache-licensed project. – amon Dec 3 at 8:21
  • @amon, changing the license terms, even when changing them to a compatible license, always requires permission from all licensors. This is something different than using the code in a work with a compatible license. – Bart van Ingen Schenau Dec 3 at 10:15
  • @BartvanIngenSchenau You're right, and I didn't phrase my comment clearly. Relicensing requires permission, but migrating to a compatible license does not (taking into account that license compatibility is often a one-way street). After the migration the old license would still stick around for attribution reasons, but would no longer be relevant for recipients. E.g. a MIT → Apache migration could place the old MIT notice into the NOTICE file. – amon Dec 3 at 10:24
  • Conversation moved to: opensource.stackexchange.com/questions/9077/… – Michael yesterday