How hard could it be to modify the Panzer IV D/E kit into something earlier like a Panzer IV A, B, or C.
The Pz IV B and C are very similar beasts. If you had a kit for one, you just about have a kit for the other.
- Same hull, from the treads to the track guards. Nearly identical placement of tools and spares on the guards as well. Same turret (except the coaxial MG has an armored sleeve surrounding the barrel on the C model). Same driver and radio operator hatches (they line up, because both models have a straight front armor panel). Nearly identical rear deck (the locking mechanism on the deck hatches was modified from round on the B, to rectangle on the C).
And the "cool bit" is that only 42 Bs were made. The first 5 used the A hull, and the last 30 used the C hull. So really, only 7 B models were built using the B hull. So if you had a C model kit, and replaced the shrouded coaxial MG sleeve with a regular coaxial barrel, you would have 30 of the 42 B models!
Production of the B ran May to October 1938. A feature of later models that was added retr0-fitting was the welded rain guard over the driver vision portal. That started in Spring of 1939. The C model was built October 1938 to October 1939, and here, too, they started retro-fitting that welded rain guard to earlier produced models in Spring 1939.
So even that subtle difference between a B and C (when first built) is gone by Spring 1939, when earlier models were retro-fitted with the rain guard.
The D model has the angled front plate, which changes the upper deck hatch placement. So there would be work there, changing the right-front portion of the superstructure, cutting and then moving forward the radio oerator side so the hatches line up as in the B and C models. I think re-working the front plate to make a flat plate would be easy.
But placement of tools and such on the track guards changed, and you pretty much just have to live with what comes molded on whatever kit you use. No way are you filing down axes and spanners and pry bars, and then modelling them elsewhere. So your B/C would have tool placement of the D. There are differences regarding the side hull and turret hatches. And the sides of the rear deck are a bit different. Do you fiddle with those bits? Or is changing the front hull to have a flat front plate, lined up hatches, and no MG enough of a visual change to call it good?
The Pz IV A is rather a different beast. The superstructure was wider, coming out to the edge of the track guards, so there is work there making it narrower. The good news is, taking a D and making an A means you already have the angled front plate. The bad news is, the hatches are different. And there are hull and turret hatch differences as well.
I think making an A model from a D would be a lot more work than making a B/C from a D. And you will need to compromise on a lot more things that should be different (like tool placement on track guards) that would be hard to make different.