Yeah, see this thread
http://www.flyertalk.com/forum/unite...er-update.html
The way it will work is if you only have a onepass account that number survives, you don't need to do anything.
If you only have a Mileage Plus account, they will assign you a new onepass number
If you have both, you should link your accounts so they know which account is linked to the other, and the onepass one will survive.