Yes, but the PNR must be divided first. Only one class of service is allowed for each flight segment, and applies to all people in that PNR. The person who is changing to a different class of service (Z instead of Y, or U instead of B) must be split off and put in another PNR by themselves.

Another note to add to this situation: the OP was not clear when he/she stated that ONE WBC seat had opened up. Did he/she mean one seat opened up for an upgrade, or that there was now one seat available and all of the rest were already filled? Most assuredly, if there was one WBC seat left on the entire plane, it would NOT be available for a mileage upgrade, but would be held open for sale until the last minute, in hopes of some passenger showing up with thousands of $$$ and an immediate need to buy a ticket on that flight. On the other hand, if there were several empty seats and one had suddenly been converted to a mileage upgrade seat, then there are a lot of possibilities! A passenger with Elite Status just "might" be able to get another seat opened up for a mileage upgrade also, in order for both him and his companion to upgrade with miles. You have to ask the right questions when speaking to the right person, but this can and does happen all of the time! Best chances, of course, are when there are a LOT of empty seats and when the departure time of the flight is imminent. Further considerations are how high your Elite Status is, and how high is the fare that you paid.

If you don't ask, you will never know what might have been possible!