idontknow,
We are SkyMiles members and I had received vouchers on other tickets, which, as you know, can only be used by purchasing over the phone. Therefore, no 1000 bonus miles for booking online

. But hey, a credit is a credit! I think this REFUND had to have been in error. The only downside is that I don't have a $200+ credit to use as an excuse for another trip to WDW! LOL
erikthewise,
There is no time limitation, however, the lower fare must still be available when you call. One customer service rep. at Delta also implied that the credit can only be issued on their published fare, and not their internet specials. But bascially it works like this: Let's say you purchased 4 tickets @ $200 on February 1st for a flight on July 1st. In April, there is a promotion and the same EXACT itinerary that you purchased is now being sold @ $150. They only allocate so many seats at a particular fare; the flight may have 50 available seats but only selling 30 at the discounted rate. When you call for a credit they would issue you a $50 voucher for each ticket. They would still have 50 open seats on the flight, but only 26 discounted fares. And that voucher is only good for one year from the ISSUE date of the original purchase. In this example, you have until Feb 1st of the following year to TRAVEL on other tickets purchased with that credit. Additionally, if they only had 2 seats left at the discounted fare (though 22 open seats!), they would only be able to credit you on 2 of your tickets, IF they even tell you that there is a credit available!
I know I am rambling here, but there have been many times that my fare searches turn up one price for one passenger and then a higher price for multiple passengers! Such was the case for the flight that I received the $58 credits (refunds!) and this was where I learned how to play the airfare game. The tickets are for holiday travel (Thanksgiving) and when I searched for a single passenger the fare was $247, but when I entered 4 passengers it was $267 each! I was able to buy as many as 3 tickets @ $247, but it would have been @ $267 if I bought ALL 4 at the same time. So I bought 3 tickets, then 1 ticket separately! If I had bought all 4 at once @ $267, there would still be 3 tickets at available for someone else to purchase @ $247.
To further complicate things, when the fare was initially lowered, by the time I checked it there was only 1 seat left (@ $207). Now knowing how the airfare game works, when I called for a credit I had it applied to the $267 ticket. I didn't even try to ask for a credit for all 4 because I was certain to be told that the discounted fare was sold out. Then the fare was reduced even further @ $189, and was available all 4 tickets! So in the end, I received three $58 REFUNDs, one $18 REFUND and one $60 CREDIT.
Sorry for the long post but I hope this helped.