What
@Wood Nymph said. AP discounts act like other Disney discounts....they come out at various times of the year, cover certain dates, and tend to vary by season. AP discounts often (but not always) come out after general public and other types of discounts, but they have longer "book by" windows than other discount types. Disney's standard "the number of rooms allocated for this offer is limited" disclaimer is very much in effect.
So yes, you can apply an AP discount to an existing rack rate reservation if AP discount inventory for that particular room category is available. It's quite easy to do online.