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Since asking this question I've updated to 12.2 (Monterey), but it's the same problem on either.

I was nervous about blowing away my existing `brew` installation, even if I first captured a list of packages that I'd need to reinstall.  I found a [blog post](https://blog.smittytone.net/2021/02/07/how-to-migrate-to-native-homebrew-on-an-m1-mac/) about installing the M1 version of `brew` alongside my existing one.  It turns out to be straightforward: the older version installed into `/user/local/bin` while the new one installs into `/opt/homebrew`, so they don't compete with each other.  After updating PATH I can use the new one by default but still have access to the old one if I should need it.  I defined an `oldbrew` alias, in part for the convenience and in part as a reminder to future-me that I still have that lying around.  If, in the future, I go through old-brew's packages and install them all using new-brew, then it'll be safe to remove the old one.  Until then, as far as I can tell, nothing's broken.

(Unfortunately, updating `brew` did not solve the Ruby problem that led me to replace `brew` in the first place.  So this might have been a red herring, but it's probably better to be on the version of `brew` that's designed for my current hardware.)