Tim;
There were a few IV-D that were "up-gunned" with the 7.5cm L/43... This variant has been documented as being used in training and some in combat service, and has been kitted by Dragon in the super-nice Pz.Kpfw IV ausf D "up-gunned" kit 6330.
There is one preserved example of the up-gunned D, at Bovington UK. This specimen was collected by the Brits from a training base after war ended.
It should be noted that the up-gunned D came out after the L/43 gun was developed and mounted in IV-F2 and G. The up-gunned D was not a developmental test-platform, but rather a way to use some old D tanks in the prevailing mid-war long-gun environment and circumstances. The up-gunned D also usually appeared with the turret schurzen that became standard IV installation only with the mid-production and later Ausf G - just as seen on the Bovington tank.
Use of D up-gunned in training allowed NSKK to train crew in long-gun IV operation without tying up the badly-needed G tanks. A small number of these up-gunned D's also served in Italy with 26th PzD in early - mid 1944. (A case of "send us something - anything", I think).
Despite the interesting Dragon box-art showing such up-gunned D in a Russian Front setting with a Polikarpov I-16 buzzing by, there seems little evidence that up-gunned D actually went to Russia - but having said that, there COULD have been such, somewhere! History definitely records the growing need for anything with a big gun on it for the German side from 1943 onwards. Dragon provides both a paint-scheme and perhaps markings for a "Russian Front D" in the kit. I just haven't seen this operational claim well-substantiated!
Bob