The second point isn't relevant to bump-formula-pr specifically. (I do think bump-formula-pr and boneyard-formula-pr should run update with --preinstall, but didn't bother to submit a PR.)
But update --preinstall happens to most users all the time. It has a much larger impact than anything bump-formula-pr does.
This is very different from Homebrew/homebrew-core#9316 in that I'm not asking about prompts here (I talked about consent, but what I meant was consent given by explicitly issuing an install/uninstall command), or arguing that update should not perform migrations. What I'm calling into question are
1. When a migration is performed, **specifically a formula => cask migration**, `brew uninstall --force` is called on the formula, which is both unnecessary and destructive, and does not happen in any other circumstance.
My suggestion: only call `brew unlink` on the formula.
2. New formula/cask may be installed by `brew update --preinstall`.
My suggestion: not install new formula/cask in `brew update --preinstall`.