Originally Posted by
anbhc
Delta Twitter: Unfortunately, please kindly note the Global certificates only apply to Delta operated flights, so it would not be eligible on the MU flight
DL Twitter is technically correct but if the IRROPs booking engine will place you into MU J, there's no GUC to apply. It appears to be treating you as a displaced J passenger (which is appropriate since your GUC has already cleared). No different than approaching the counter if the flight was canceled or delayed and having them move you to MU but honoring the GUC cleared instead of downgrading you.
Just make certain that when you click through it is actually booking you into Business since it has major caveats about being a different class of service (presumably being flagged since the system sees "Business" as a different parameter than "Delta One")