[terramex]

No, in this hypothetical case you would be fine. Mimemagic case is more like 15 years ago you had a MIT codebase that contained hidden GPL-derived code. One day you are notified about that violation and relicense whole codebase to GPL, but the code that was already in the repository before has never been truly MIT - it was always violating GPL. To fix this you can either relicense whole codebase to GPL (what mimemagic chose) or remove GPL code from codebase, stop distribution of older versions and continue your project as MIT.

Going back to older version of code does not change anything as now everybody is aware of violation so you don't even have plausible deniability. You would need to fork from commit before adding GPL code, but this is impossible for mimemagic as it contained this code since day one.