I think it's definitely luck - being in the right place at the right time.
From what I've read here, upgrades seem to be done for the resort's convenience, and are largely assigned ahead of time. If they need to even out the room counts for the housekeepers, or free up a block of rooms for a convention, or take rooms out of service for maintenance, they may upgrade some people to more expensive rooms. This is all figured out and done before you even arrive.
So, upgrades do happen, but they're not as common as they appear from reading these boards. Remember that the concierge rooms at a deluxe are a small percentage of the total rooms in the resort, and most of them are booked by paying customers. So there just aren't that many rooms available for upgrades.
It's not something I'd get my hopes up about. You can ask when you check in, but I think it's more than likely that they'll tell you what it would cost to upgrade instead of offering it for free.