This worked for me. YMMV
brew uninstall --purge macvim
brew install macvim
[amended] Regarding the upgrade alone option, that may work if you don’t have the current version already. The --purge forces a removal and complete reinstall.
Also, the build from source option, while it may work, you now own your own mac-vim static snapshot and need to maintain it manually as the app evolves.
I’d avoid building from source unless you have very specific needs.