As data points (I am a CO Plat), I was flying BOI-MSP (2 PM Departure) a couple of weeks ago. The flight was P6F6. At 23:59, it asked if I wanted to upgrade and when I said yes, I was told I was put on the waiting list. I aborted the process.

I tried at 7:30 AM and got the same response and aborted.

At about 10:45 AM I tried again and this time, after answering Yes, I was assigned a FC seat.