Another data point: upgraded to D1 on SEA-NRT return 3 weeks out. Originally was an award booking (in coach) as I wasn't sure if I was going to take this trip. Cancelled it and bought a coach fare + GUC. Much better deal this way. Coach fare was $1400. Award ticket was 150,000 in coach, 325,000 in D1.
Double bonus was my GUCs expired on the day of the return leg. First year I've been able to use all of them (in fact, I got an extra - on ATL-JNB last year they did not process my GUC on upgrade at the gate properly
)