Near as I can remember, an automatic runs on a fluidics principle with a set
of springs and valves that are controlled by fluid pressure (rpm), throttle
position, and the gear selector arm.  A suitable amount of gunkystuff in the
transmission itself can prevent the downshift feature from working correctly
along with any of the other valve controlled features.  If you flush, not
drain and refill, flush the transmission and clean out the screen an the
transmission cooler to get rid of all the collected goo, the transmission
will work as well as the clutches will allow.  The torque converter is a
vane to vane device that passes torque through from the input vanes to the
output vanes via the transmission fluid.

A proper flush will restore most of the features except for the harsh
shifting that may be caused by worn clutches.  In neither case is the torque
converter at fault unless the bearings in the converter are excessively worn
or the seal is compromised.

It's pretty easy to "compromise" the seal by intentional or unintentional
damage when de and re installing.

A reputable shop will not install parts that are not needed.

Jeff Field may have comments on this and I've copied him on the original.

p