Airbus recently came out with an extended life cycle program for the 320 series which is probably the reason that UCH is re-thinking the timeline. Prior to this the hour/cycle restriction on the 320 was a hard limit and the reason for the "disposable aircraft" moniker. There was a running change during production so only 320's built after a certain serial number are eligible for the extension. This is presumably the reason that DAL is retiring their oldest 40 (or so) Airbuses as they time out but retaining the rest. All of UAL's should be eligible for the program based on delivery dates.