It's not rare for us, we've received an upgrade 3 out of the 4 times we stayed on propertyHowever, it is rare to receive an upgrade at any Disney Resort. Last August was our twelth stay on property and the first time we have been upgraded.
the only upgrade we've ever had was 1 night in the vice predidential suite!! We had booked 2 regular rooms for 1 night and the rest in a 2 bedroom villa, those with DVC points. We checked in early and no room, checked back at 4--no room, at 5, checked and they said we would be escorted to our room by the concierge--almost fell over when we saw what room we had.