The GM units come ready to plug and play, no adapt relearns or programming needed. If all of your downshifts are harsh, it can be your tuning and/or non-transmission causes, as also suggested. Engine/powertrain sensors and switches can also cause irregularities in transmission behavior. These include the MAF, TPS, Ignition switch/module and many more.
These symptoms can also be caused by weakness in the transmission itself, specifically the compensator feed system which is a combination of apply, accumulation and exhaust circuits designed to mechanically control shift timing, firmness as well as prevent centrifugal apply of clutch packs inside the unit when they are not supposed to be on/working. It can also be the torque converter failing which, in case of it being so new, it would be defective.
If it's just the 3-2 downshift that indicates to me something's potentially weak in the 3-5-R hydraulic circuit causing the clutch not to release timely. This can also be cauesed by the TEHCM or valve body (or both).
I'd rule out any non-transmission causes first then contact a GM service center and see what they will do for disgnosis and warranty replacement if/as necessary.
Whichever shop or dealership svc center did the trans R&R work should have also inspected all mounts, the driveline, wheel bearings, flexplate and other systems underneath to catch and repair/replace anything that's worn/bad. They should have also done the rear main seal, flushed your lines and transmission cooler(s) to ensure all contaminant from the previous trans failure was removed.
