A few years ago we were upgraded from Pop to AKL Arusha Savannah view with bunkbeds. We checked in in the late afternoon/early evening in February, and when I checked in at Pop she took our info, turned to another CM in the area and asked him if they were still walking guests, made a call, and asked me if we wanted to stay at the AKL. Stupidly it took me a minute to decide - I guess I was taken by surprise so I had to think about it! Thankfully I came to my senses and said yes, assuming we would get a standard view room at the AKL. I was shocked to see what we did get, and it was great. I don't dislike Pop at all, but the AKL is light years better to us and the kids loved seeing the animals from the balcony in their PJs.
Our only other upgrades over many trips have been to standard to water view at POR, preferred room instead of standard on anther Pop trip, garden view CL instead of SV CL at the YC and TPV CL instead of LV CL at the Poly. Nothing like the jump from Value to Deluxe we had that one trip! Oh, I forgot - when I was still traveling with my parents and WL was new, we were called ahead of time and offered WL instead of POR (then DL), which we took and loved.
ETA the AKL upgrade was for 8 nights and the WL upgrade was for 7 or 8 nights, which made them even better and more amazing!