That kind of upgrade is extraordinarily rare. A complimentary upgrade within a resort - say, from standard to club level - is highly unusual. An upgrade to an entirely different resort is about as rare as hen's teeth, and would be due to a completely overbooked resort, or some kind of maintenance situation that affects a significant number of rooms. Or part of a resort being shut down for a refurb.
Remember, there are something like 30,000 rooms on Disney property. Thousands of people checking in every day. All but a very tiny percentage of those people get exactly the room category they booked. The handful of reports of resort upgrades you read about on these boards is a very small drop in the bucket.