Based on previous upgrades experiences at ORY, including with the app, you'd get them a couple of hours before you land.
However, if no options are available or if they do not suit you, it's always best to talk to the agent at the preferred booth in the car park. they have always been pleasant and accommodating with my needs. As a Pc it has always been a one-class upgrade at least, and up to 4 categories at best (daily rental, rented a Peugeot 208 got a C-class)
IIRC the only time when I could not get another car was on a Friday afternoon during holiday season, only one car in the rented category (SUV) was in the car park - mine, and the other options were one Fiat 500 and a Renault Clio.