Six of us were booked on UA 42 in December, couple months away. I checked the flight and it is now all fully booked, then checked the seat map, only 30% assigned. Is it an early sign of a cancelled flight? All other flights are widely available throughout the day, and even days before and after. It is just odd for that one flight to be sold out like this.

1*A#UA1674 F9 C9 A9 D9 Z7 P7 Y9 B9 SFOIAH 415P 955P 6 738 0E
M9 E9 U9 H9 Q9 V9 W9 S9 T9 L9 K9 G9 N9

2*A#UA 42 F0 C0 A0 D0 Z0 P0 Y0 B0 SFOIAH 530P1116P 8 738 0E
M0 E0 U0 H0 Q0 V0 W0 S0 T0 L0 K0 G0 N0


3*A#UA 20 F9 C9 A9 D9 Z9 P9 Y9 B9 SFOIAH 220P 806P 739 0E
M9 E9 U9 H9 Q9 V9 W9 S9 T9 L0 K0 G0 N0
4*A#UA1589 F9 C9 A9 D9 Z9 P9 Y9 B9 SFOIAH 150P 744P 738 0E
M9 E9 U9 H9 Q9 V9 W9 S9 T9 L9 K9 G9 N9
5*A#UA1474 F9 C9 A9 D9 Z9 P9 Y9 B9 SFOIAH 730P 104A#1 73G 0E
M9 E9 U9 H9 Q9 V9 W9 S9 T9 L9 K9 G9 N9
6*A#UA1604 F7 C7 A7 D7 Z5 P4 Y9 B9 SFOIAH 1200N 550P 738 0E
M9 E9 U9 H9 Q9 V9 W0 S0 T0 L0 K0 G0 N0

What route is this?

I've seen exactly that happen before: they zero out a flight well before it's fully booked, and then end up canceling it. I think they do this when they think there's a pretty good chance that they'll cancel it, but aren't quite ready to make it official and permanent. In my case, it was for a flight a week or two out, so the cancellation came within a day or two of the zero-out, but I'd imagine it could be quite a bit longer in the case of a flight that's months away.

It shows SFO-IAH.

It's very likely this will be cancelled. It will probably load tomorrow night with schedule changes.

Once it is, UA will rebook you on another flight.

If you don't like the rebook, you can ask for something else. If the new flight is materially different in time, you can get a refund.

If fares have gone down, refunding may be preferred.

or aircraft change

Thinking about December, it could be a bulk-travel booking too -- university group or something. I have family members that have chaperoned a couple of these, where you'll have a 300-person international travel party broken up across domestic flights and airports with 100 on one flight, 80 on another, 60 on a third etc. heading toward the international hub.

I can see how a group will book the entire flight in economy class, but it is just odd for a group to book all First class seats too?!

It's not a group booking. UA would still be willing to oversell. It's a cancel or an equipment change. (Or an IT bug - it almost goes without saying that's always a possibility.)

Same thing happened here.

Dec. 24 BOS-IAD UA 363 7:15p-8:58p

All zeroed out when seatmap shows largely empty plane.

There are only 4 flights a day BOS-IAD. This is the only evening flight.

IT bug?

Either it will be canceled, or it's an aircraft/flight number change. I'm betting on the latter. At this point, there are plenty other alternatives, so I'd just keep an eye on it for the next few days.

For Christmas Eve, I venture to say that this will be canceled.

Ouch.

I am connecting BOS-IAD 7:15p-8:58p to the 10:15pm IAD-DXB flight on Christmas Eve.

The earlier flight is 2:06p-4:04p which means 6 hour layover at IAD.

Can it still be booked?

Reread the OP; looks like it can't be booked. I had a DEN--BWI or IAH--BWI flight behave that way earlier this year, and it ended up being dropped from the schedule.

The flight is now gone! we all got rebooked to an earlier flight!