> I'm surprised they went with a all-at-once reformat. Even when doing it over a weekend this is bound to mess with a lot of open PRs at their scale.

Rebasing PRs should be trivial, just rewrite all commits reformatting the files it touches, then rebase, `git checkout --theirs`, run formatter again, and `git rebase --continue`. It's methodical and scriptable, you don't need to manually resolve any conflicts.