First off, the 50K RT is what biz costs.
Second, mileage is probably the best way to go, because its much more changeable than revenue tickets.
AA has a very generous change policy in that they allow free changes for a year, as long as Origin and Destination (OandD) are the same, IIRC.
Continental (MR) also has the same, with the exception that if anything (original date, new date) is within 21 days they charge $75. They also let you book award seats at the 50K for Biz level on US and UA.
Regarding availability, GL.