I am one of the maintainers of aiosmtpd [1], and the largest PR I ever made was migrating the library's tests from nosetest to pytest. Before doing that, though, I discussed with the other maintainers if such a migration is welcome. And after getting support from them, I made the changes with gusto. It took weeks, even months to complete and the PR is massive [2]

But still the crux of the matter is: Massive changes require buy-in from other maintainers BEFORE the changes even start.

[1] https://github.com/aio-libs/aiosmtpd [2] https://github.com/aio-libs/aiosmtpd/pull/202