I just did an edit for the Selestia base where I swapped her head with Shinra's head. I did everything in blender. It's probably best to do it in blender as well because you need to adjust the head just right and sculpt around the neck to clean it up otherwise you will see the obvious cut between the head and body mesh. It's important to get rid of all bones related to the original head because it will only confuse Unity later down the line. I will also highly suggest you do as much as you can in blender because you can clean things up a lot easier in blender and make sure everything get parented. For some reason the converse never wants to work properly in unity so I've been pulling the fbx from all the prefabs into blender instead, gives you a chance to customize blendshapes as well.
I might actually be able to figure this out because I just came across an avatar last night that looked exactly like one of those, my friend said he would send a link to where he got it, apparently off some guy's gumroad, it should have info on what bases he used, etc. I'll post as soon as I get the link, hopefully it helps lol
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