HEAD formulae can't be upgraded by just running
brew upgrade <formula> --HEAD. The only way to upgrade
HEAD formula is to uninstall it and install once again or using
reinstall which is almost the same thing. This is not very convenient. We don't know if it's time to upgrade formula, we don't have other versions. We don't reinstall stable formula to upgrade them and the same way we don't need to reinstall head formula to upgrade them. The outcome of the project is to provide
HEAD versions and the ability to upgrade them.