You're talking two different processes. First is the upgrade, and that's to a cabin. Seat assignment is a different (second) process. After you get an upgrade, UA assigns a seat - you are NOT required to stay in that seat if others are available. If you don't like the seats assigned, others may be available for you to choose and others may free up as booked passengers cancel, pick other seats, or mis-connect. Unless you're on a 772HD, not sure middle seats should be a problem. And you'd rather sit in a more cramped seats than more comfortable seats just to be together for a single flight?

If you insist on downgrading, you'll have to call. It cannot be accomplished online.