Transmission flushes do not really need to be done.
Actually when a transmission flush is performed sometimes it can do more damage to your transmission than if you hadn't had the transmission flush in the first place.
If you're gonna change out your transmission fluid with new transmission fluid you should get the transmission fluid completely drained and have the filter along with the transmission pan gasket changed all at once.
That would be the best way of changing your transmission fluid since the filter will be changed at the same time.