Air Asia is one of the delights of Southeast Asia travel, making many destinations affordable in a sassy package.
The two airports are not close together and transfers will not be convenient. There are no rail links; road traffic in Bangkok is occasionally light between midnight and 5 am, the rest of the time, good luck.
For those with tickets in hand they are allowing changes, from the press release:
AirAsia has scheduled October 1st this year as the date when all of its services will be based at Don Mueang Airport. The airline’s flights will retain their FD code and original schedules. Passengers who are affected by the switch to Don Mueang Airport, may alter their flights by either choosing another travel date on the same route either 7 days before or after their original travel date, at no additional cost. Passengers also have an option to convert the paid flight into credit shell available to be used within the next 90 days. The change options are only available to affected passengers who purchased their tickets before 26 June 2012 whose travel dates are from 1 October 2012 onwards. For further information and developments visit www.airasia.com or contact the AirAsia Call Center at 02-515-999.
Don Mueang is famous for its Royal Thai Air Force Golf Course, Wikipedia dryly noting:
An RTAF golf course is located between the two runways. The course has no separation from the runway, and golfers are held back by a red light whenever planes land.