What resort are you staying in?
Typically upgrades are given within the same resort - like you're staying at Riverside with a standard view and the front desk upgrades you to a water view for no charge.
On rare occasions when a resort is full/overbooked you may get upgraded to another resort, but this is very rare.
From what I have gathered it seems some resorts are more apt to upgrade than others - AKL has fewer standard view rooms than the other views so if they overbook you have more of a chance of being upgraded to a better view. I've also read that its the policy of YC/BC to upgrade whenever a better room is available, but I don't know how accurate this is and it would still be hit or miss as to what is available when you check in.
My feeling has always been if a better room is available the front desk will offer it, but some people ask politely if anything is open for a complimentary upgrade.
Hope that makes sense!
Melissa