Ask nicely
One of three things will happen.
1. no upgrades are available.
2. An upgrade is available if you'd like to pay for it (sometimes at a discounted rate)
3. You'll be given a free upgrade.
At WDW, number two is generally the rule of thumb, although sometimes people do get lucky!
Bottom line, book what you'll be happy with and consider it pixie dust if you get the upgrade
Anne