When there is a conflict, brew upgrade will stop. For this particular case, I will use parallel from the gnu parallel package instead of from moreutils. When I originally installed them, I installed moreutils first then force install parallel. Can upgrade for this case be automated? I don't want to always do this manually.
==> Upgrading parallel
Error: Cannot install parallel because conflicting formulae are installed.
moreutils: because both install a `parallel` executable.
Please `brew unlink moreutils` before continuing.
Unlinking removes a formula's symlinks from /usr/local. You can
link the formula again after the install finishes. You can --force this
install, but the build may fail or cause obscure side effects in the
resulting software.