Happened to me also when I changed my ticket cuz a cheaper fare became available in the 24 hour window. The rep just refunded me the difference in fare instead of reissuing me a whole new one ticket. Somehow, the ec purchase was also refunded. When I asked to pay for it, he said there was no place for him to enter the price, so he just kept me in my original ec seat for free . If I try to change the seat at delta.com now, though, it says cost $60