On at least one occasion, the "upgrade" meant they had slippers and bathrobes in the room. On more than one occasion, an "upgrade" meant we were assigned a room on the "HHonors Floor" - which only meant there was a sign indicating that in the corridor, or maybe it was on a higher floor level. Full stop. Yes, I feel really special and "hhonored" in those cases.
Each property has their own definition - sometimes daffynition? - about what constitutes an "upgrade," and some push the envelope on the use of the word.