Domestic Itinerary on deeply discounted fare (T) for next day travel.

Both segments on regional jets.

About 22 hours before scheduled flight time 12 noon central time, did a no-charge (PLAT) change-this is called same day change, and moved to a flight that was about an hour later than the original.

New itinerary segment #2 has an upgradable aircraft. Time now, about 23 hours before flight time, of new flight.

Within an hour of the change, I get the email upgrade notification.

I thought the policy was that I would have to wait until after midnight for EUA to run. I am glad it happened, but why?