While you'll hear of someone getting a free upgrade now and then, I think they are very rare at any resort. If and when it happens, it's usually at a very slow time. I wouldn't expect to be able to upgrade upon check-in in June even for a fee, let alone for free.
The posts I've read about being upgraded for free have almost all been with guests who were just "given" the upgrade without asking. I know after reading these boards it sometimes seems like everyone gets upgraded in some way on every trip. It really doesn't happen that way....at least not for us!