Pretty sure they will be running files from Mileage Plus into the new system for a day or so and until then, Mileage Plus balances and statuses may be incorrect:

"We are finalizing our system migration, and it may take up to 24 hours for your full account to display. Be assured that all award miles, Premier qualifying miles, Premier qualifying segments and upgrades in your MileagePlus account are safe. Also, changes to your account profile are not available until the transition period is complete.

If you would like to book award travel immediately but your full mileage balance doesn’t display

Proceed with selecting your award flights on united.com and during the booking process, choose to hold your itinerary for up to 72 hours for free with our FareLockSM service. For itineraries that include partner airlines, a free 24-hour hold will be offered. This service is a temporary offer during our account transition period.

Please note that you’ll have to return to united.com to complete your purchase once your miles are available."