Our stay begins on the same day (June 18) and I was able to choose and customize these new bands this afternoon. Our last day to select MBs under the "old" system was Friday. As a precaution, however, I declined MBs last week on the chance the new system would go live before our trip.
Best guess - if the last day to customize MBs under the old system occurred before today and a guest selected bands by that date (or didn't select anything and defaulted to the standard grey bands), the order is being processed and new bands can't be selected. If a guest previously declined MBs and there is still time to customize under the new system (11 days), however, the guest can select and customize from the new bands even if the last day to customize under the old system already passed.