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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