Hard shifting gear up and down

During these days, i have noticed that during low speed driving (20~40 km) my auto gear shifting is a little bit bump and sometime hesitate in both up and down.
My car ATF is not changed yet and mileage is over 50,000 km.
I am worry of something major cracking in the auto gearbox and it cannot be repaired.
Could you please kindly give the information of cause and effect of that symptom and how i can maintain.

Thanks in advance.