The vouchers you speak of for fare differences after booking are only if the fare for the same exact itinerary go down. AS will re-issue the tickets at the lower fare and issue you a credit towards another purchase for the difference.

With your situation you are making a voluntary change. If the fare you are changing to is lower than the ticket you're changing from you forfeit the residual value. Sometimes if the difference is great the AS agents will price the new itinerary in a slightly higher fare bucket to miniize the loss. This is so that if you need to change the ticket again you have as much of the original value as possible to apply to a new ticket.

ETA: Basically what missy said...she's just quicker on the draw than I am!