Had something similar happen yesterday on flight UA526 SFO-YYZ.
It started out as a A319 with 8 seats in coach, then 3 days before flight looked like A320 with 12 seats, then 2 days before it said A319 with 8 seats, then day before it said A320 with 12 seats (and at that point, UA premier support said it was an A319 with 8 seats!?)
In the end, it was an A320 with 12 seats and I got the last upgrade (as a 1K from XN, i.e. award ticket). I asked the guy who got the second last upgrade, and he said he was 1K.
There were 9 upgrades on that flight, all GS/1K/Y-up - wow!