(if this is particularly easy for anyone please go ahead and self-assign this issue, thanks!)

https://chromium-review.googlesource.com/c/infra/infra/+/1053409 fixed it

I think that addresses a different problem than what I'm asking about.

In particular, what happens if the migration app files a bug for a builder that exists on buildbot, then the builder is renamed or deleted on buildbot? What happens to the initial bug that was filed? The migration app doesn't close any bugs so can't we have some bugs open that don't correspond to any active builders?

yes, the CL addressed a different problem, but incidentally it fixed this problem

If the builder is renamed, it is not longer present in the GetCompressedMasterJSON response, so app closes the bug. Same happens in builder is removed. The app closed bug 841082 today because this builder does not exist on buildbot https://ci.chromium.org/buildbot/chromium.android/android-kitkat-arm-rel/

If a bug is already fixed (e.g. by a human), the app's new comment with status=Fixed won't changed the status. It would, though, if a human set it to Verified, which would be a bit confusing.

Ah, ok. I was under the impression the migration app didn't close bugs anymore but I was mistaken. Thanks for fixing this!