If I'm not mistaken, if all code authors agree on the license change, then, the new release can be made fully GPL. While the old copy will be still available as MIT.

A malicious actor can pretend that they merged MIT parts with new GPL parts, but I think, it would not take a lot of time, until such merging would become technically hard, and the code can be effectively converted into GPL.

However, since Zen's guy was a contributor, it's probably not possible to get all authors' permission to change the license for the entire Zig codebase.