To be a bit more specific, your cancelled ticket will return to the inventory with its original fare class (your old award rate). It will not be priced at the new current prevailing rate. However, this all happens within a few minutes and 'your' cancelled ticket will be available to the general public. If there is another person who just so happens to search for the exact same itinerary while this happens and is faster than the agent you're calling with, they will 'snatch' your old award rate and you're left with the current prevailing rate.