9.0 does not support Standard downgrading to an older . However it is possible to rollback to the previous version by unpacking the old MZ_HOME backup that was done earlier.
It is therefore recommended that you perform a Simplified Upgrade Procedure to ensure that the existing environment is present to fall back to in case the upgrade to 9.0 fails. The steps detailed in Simplified Downgrade Procedure will guide you to restore the previous environment.