Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Well, this is not exactly what you're asking, but award tickets issued from AAdvantage miles permit you to change dates of travel without charge (as long as you can find award seats on your new date of travel). AA even lets you change carriers on an award, as long as only oneworld carriers are used on both the old and new routings. (Redeeming on BA metal will have high surcharges; redeeming on other AA partners -- like CX and QF -- will not.)
When AA runs a sale on miles, they can sometimes be bought for around 2.1 cents (U.S.) per mile.
I'm sure others will chime in shortly on flexible cash tickets.
What class of service?