What time are upgrades processed at the given window for each status level, i.e. 5 days for plat, 2 for gold etc? Are they all processed at midnight of the applicable day, the time of the actual flight or what?

I just made several reservations for next week and have lousy seats for all three segments. Most are later in the day and I would like to try and snag an exit row as quickly as possible when it comes available to accommodate upgrades.

Anyone here know if there is a set rule?