Like so many people in this forum I faced this issue too. My first advise will be "stop using your car"
I don't know how bad your problem could be but if you continue to use your car as it is, you will burn the clutch packs and your transmission will definetly will need a rebuild.
With this symptoms, what I would do first is to check the solenoids, make sure they are working properly. Solenoids tend to stick in one position, when ohm checked they are OK but they are mechanically stuck and will not move when TCM commands them to. This will be a good place to start.
Changing the fluid won't make your tranny fail, but ATF is not cheap, so make sure you don't have to drain the ATF once the new ATF is in.