When the system automatically booked you a day earlier, were you booked into economy or first? That would be a good indication on how the “system” views your upgrade. If it rebooked you directly into first, then it would have been a HUACA situation for the agent telling you that she could not book into that cabin.

I’ve personally never been rebooked on the same flight, but for schedule change rebookings, and upgrading on instruments (swu), I get the upgraded cabin unless no seats are available in any fare class on the cabin. I haven’t cleared in advance for 500-mile upgrades in a couple years, so I can’t recall the procedure for that.