Chrysler Repair: Transmission harsh downshifting, piece of dirt, dodge stratus

Question
I recently dropped the transmission pan and replaced the filter on my wife's 02 dodge stratus.  The car drove and operated fine prior to the servicing.  Upon completion of the filter change and replacement of ATF 4 fluid the car shifts hard in deceleration roughly 20 mph as I come to a stop.  Not sure why.  The transmission operates as smooth as glass from 1st thru 4th and then back down 4 thru 2nd.  It's just 2nd to 1st gear that it jerks into gear.  I have dropped the pan a second time to make sure the filter was on correctly and double check everything.  Found no problems or errors visible to the eye.  Not other work was done.  I did not flush, wipe or air blast the exposed transmission and can not think of what may be causing the problem.

Answer
Hi Rolf,
Aside from a coincidental damage to an internal engine part, the possible causes of hard down-shifting are: too low fluid level, aerated fluid, hydraulic pressure too high, engine idle speed too high, or valve body malfunction/leakage. Ideally one of those is the answer and so it doesn't involve anything that requires removal and rebuilding. Let me know what you find, please.
I wonder if a piece of dirt may have gotten into the valve body? You could get a fault code readout for free at an Autozone parts store and see if any 4-digit fault codes show up related to the transimssion. If so let me know and we'll go from there.
Roland
PS Sorry for the delay but I just found your question in the pool to which it had been referred by the other espert. I would appreciate your 'rating' my answer and where you see the question about 'volunteer of the month' consider giving me a 'yes'. Thank you.