This has been done before. There was a "mistake" B-fare ($650 i believe) SEA to NRT some time back on CO. A lot of people took up that one.

In NRT, you just stay in the sterile area for transit. You do not need to go to immigration at all.
You will have to re-clear security. So the rules for liquids still apply.

As soon as you clear security, you will see the CO desk (and United as well) on your left. Get your new BP, and get back on that plane!

If you have time, there is the UA RCC right past the ticket desks.
(close to CO and UA gates).

EDA: Note, I myself have not done this! Dont want to do 2 super long flights back-to-back!