Century Transmission problem

I have a 1993 Buick Century with the 3.3 liter engine and have noticed that my transmission is real slow first thing each morning to shift from 1st-2nd gear. It doesn't shift until 25-30 mph. Then slow third as well and never seems to go in overdrive now. I have noticed the mpg drop 5 mpg. Would a transmission flush and then new filter and new fluid help? Do I dare take it and get the transmission tested? I never get fourth gear. Once the car/tranny is warmed up, the changes are much smoother but still not seeing fourth gear. Thanks.

Paul