Let's assume you upgrade from package/foo-1.0 to package/foo-1.1. You can then use quickpkg to create a backup of foo-1.0, upgrade with
emerge -ua foo, and test it. If you are not satisfied, mask the updated version in /etc/portage/package.mask, and
emerge -uak foo again in order to downgrade. -k tells emerge to use the binary package built with quickpkg instead of re-compiling.
# Make binary package quickpkg foo # (or quickpkg package/foo-1.0) # Upgrade foo emerge -ua foo # Make new version echo ">=package/foo-1.1" >> /etc/portage/package.mask # Restore binary version emerge -uak foo