1990 Cadillac EL Dorado transmission problem code

My 90 Eldorado, 86,000 miles, drives fine, but the service engine light is on and I am told there is a "hard code" indicating transmission problems. A trans shop said the solenoids are going bad and it needs to be overhauled. I asked if changing the fluid would help (it hasn't been changed in a very long time and I live in hot Phoenix, AZ) and they refused to do it, saying it would make the problem worse. Can you tell me if changing the fluid might help? The value of the car makes the overhaul not worth it.