I tend to think (as previously mentioned) that short stays tend to get an upgrade. I'd love to know if the people who got major upgrades, were indeed given those for a week long stay.
Not sure if anyone ever answered you, but yes, I received an upgrade on an 8 night stay at AKL in 2017.
Original Reservation: Standard View 2Q HA Roll-In Shower Room for 3 adults
Upgrade: 2 Rooms (1 K HA Roll-in Shower/ 1 1Q 1BB) Savannah View Club Level
In this case the upgrade was given due to our original room not being available. I had checked in online and the room ready was texted to me on the drive. Since I was driving, I only knew we had been upgraded from Standard to Savannah since my aunt and sister did not know where to look to check room type. They just had a map of where the room was. We found out it was CL when a CM meet us as we pulled up to unload our car.
FYI: Same trip my Cousin's family was upgraded from a Standard View to Savannah View. I had not connected our stays since I thought their upgrade chances would increase since they did not need a special room type.
Last year, I booked 2 rooms at POFQ last minute for my parents, sister and I as we arrived a day early before a BLT check in. I booked both standard rooms, but my parents had a River View room upgrade and my sister and I had a standard.
I tend to book what I am wanting so I have not really worried about upgrades, though nice when they occur.