when should the transmission fluid be changed or flushed?

without knowing a more complete history of the trans service , I would strongly advise AGAINST 'flushing' that transmission!! Have the 'basic drain , replace filter , and refill to proper level' VERY important to use the CORRECT fluid!! High mileage , it may be 'pickier' and the internals may not like the wrong fluid put in!! IF IN DOUBT , go to the dealer and buy the correct fluid there!! (even if the price is triple , isn't that still less than a transmission rebuild?

DO NOT have the transmission FLUSHED! .. Drain fluid, replace filter & refill .. Don't know how many miles "not been changed in a while" actually is! ..... I also agree with zippie1.