This one's easier to explain. Does it happen a lot?

Punchline up front:
ABQ-IAD A319 was swapped out for an A320. I know this happens sometimes.

The story:
-Flight in Sept.
-F & NC zeroed out several weeks ago -- my UG confirmed, Ms. FCG out of luck. (Actually, I'm out of luck, particularly since she's expecting. )
-Sometime after OLCI, I'm talking to reservations about a different leg and they say her UA782 UG just cleared. But seats can't be assigned. Ask at airport.
-Check-in agent says C is full, but "you're confirmed. Ask at the gate. Here's your BP with UPGRADE CONFIRMED on it."
-Gate calls us up, says "C is full," you're back in coach. Rips up the Upgrade Confirmed coupon.

After boarding, we see 4 empty / extra C seats due to the equipment change.

Before closing the door, there's a heated debate at the front (heard by my wife) between the purser and gate agent. The GA "still sees a 319" and refuses/can't move anyone. The purser says "well, I flew in on this plane last night. You can see the empty seats." Eventually the gate agent leaves, they close the door, and the purser quietly escorts 4 of us into C.

Very nice treatment on the plane. Less so by ground staff. All worked out fine though!

-fcg