United has said that the back end will retain both numbers because of all the different credit card, shopping, dining, etc programs out there.
I don't know if that means you will or won't have to change your login information to the rewards network or not, but they will cross reference the accounts (assuming they can match the accounts if you did not link them already) for quite some time.
From UAInsider answering a question in the United Forum
How will this affect Mileage Plus Dining? And our Chase cards? And Mileage Plus Shopping? And all of the partner programs that are used to credit (e.g. hotel, car rental)? Will we have to change how those are mapped? Or will UA continue to monitor our old UA numbers and credit to our OP number? (Maybe notifying us to change the mapping when they see credit to those accounts?)
Hi bmvaughn, until our systems are integrated, everything will continue to work as is does today. After that, we'll handle the mapping of account numbers for quite some time. For example, if a car company sends us your Mileage Plus number for a rental you’ve made, we’ll have this mapped to your new number for earning purposes. That said, you’ll be safe to change your number with any partners as well (but, only after our transition to a single system takes place).