Swapping models can lead to many things, including this one. I just swapped mdf/mesh files, and you can see the result.

Let's me share how I did the model swap.
 
I swapped mesh.1808312334 & mdf2.10 files. I found out that no matter how we change the location on those files, in game the engine always let those files load the tex files from their original location, so that we can replace the mesh.1808312334 & mdf2.10 files of the zombie files by the human ones, and the game will trace and load the tex files in the original human folders.
 
Now lets look at models. For zombies, the model include FOUR sets of mesh.1808312334 & mdf2.10 files, one for body, one for head+hair, one for shirt, and one for pants. All the variants of zombies are based on combination of different head, shirts and pants.
For humans, THREE sets of mesh.1808312334 & mdf2.10 files, body (cloth included), head, and hair.
 
The files of zombies are in
natives\x64\sectionroot\character\enemy\
 
the files of humans are in
natives\x64\sectionroot\character\player
 
The swapping part is to replace zombie mesh/mdf files with human ones. We need to remember, there are FOUR sets of mesh/mdf for Zombies, THREE mesh/mdf sets for humans. IT DOESN'T MATTER WHICH BODY PART FILE REPLACE WHICH PART, you can have human hair replace zombie pants, or human body replace zombie head, doesn't matter. The real problem is what to do with the extra set. My recommendation is to use the files from invisible mod, the mod that make enemy "invisible" by replacing the body part by bullet, which is too small to notice. https://www.nexusmods.com/residentevil22019/mods/35
 
One thing you will notice, the new zombies will have disproportional body shape, which is caused by difference in skeletons.
 
Human skeletons, including male, female, and children, are in
natives\x64\sectionroot\character\player
 
The zombie skeleton is in
natives\x64\sectionroot\character\enemy\em0000
 
Rename a human skeleton and place it the place of the zombie one, then you can have a swapped zombie with normal body proportion. But keep in mind, one zombie skeleton file controls ALL zombies, so if you just swapped, for example, female zombie and placed the new skeleton file, you will find un-swapped zombies have disproportional body shapes.
 
As for how to locate the right human/enemy files, check here
http://residentevilmodding.boards.net/thread/9862/re2-ultimate-file-list

Using the method I showed above, we can even think of diversifying zombies. Apart from the default male/female/fat zombie types, all the diversity of zombies are based on different combination of body, head, shirts, pants. Since IT DOESN'T MATTER WHICH BODY PART FILE REPLACE WHICH PART, we can have something like different Claire heads in different zombie face subfolders, different hairs in different zombie shirt subfolders, different Claire costumes in different zombie pants subfolders, and a bullet replace zombie body subfolders. Then we can find diversified zombies. 
 
Also technically ivy model can also be swapped using the same method, the only problem is that ivy only got one part for the whole body, and if you want to just swap without any modeling, you can end up with headless body or a floating head.
 
Also to be notices, for some reason, most Claire models can be swapped in to zombies, but most Ada body models don't display properly on zombies, usually show no flesh, just floating clothes and shoes.

I chose to publish the one with model from arison_c because he got great models, and he is perfectly fine with me to do whatever I want with his public version mod. 

You can support him and get his mods directly here
https://www.nexusmods.com/residentevil22019/users/96834
https://www.patreon.com/arison_c

I hope more people can learn the method and create their own dream version of zombies. Nowadays many good models are paid ones, and sharing zombie swap mods based them is problematic. Some people can even make sexy Leon wandering around if they want.