Consider the Kerla kernel. It's dual-licensed under the Apache 2.0 and MIT licenses. If I were to create a fork of that, would I be allowed to change my fork's license to the GPL? Until today, I was very confident that the answer to this was "obviously yes", but in this Reddit thread, the consensus seems to be that this isn't allowed, and I have no idea why. What am I missing here? Isn't the whole point of permissive licenses that relicensing is allowed?
I read through the entire part of the Reddit thread related to 'permissive license' and I found that there is a lot of emotions and very few facts in the discussion.
The answer to your question is: Yes, you can re-license a fork. There is a lengthy page listing many licenses and their compatibility with various versions of GPL. I recommend you read that and find the frame conditions for what you want to do.
'Forking' is covered by the terms of service of whatever git platform is used. You could 'fork' a fully copyrighted codebase if the author signed up to a public git hosting service where they gave that permission. The right to relicense that fork for general conveyancing is (plausibly) implied by some permissive licenses but to be sure it would have to be made explicit in the license and MIT only offers the right to sublicense, which is about offering the software under more restrictions, not less. Permission to relicense work from the original authors under MIT I think is a highly opinionated implication - it depends on who you are talking to within the industry or company, since it's not been a matter of case law AFAIK.