Skinned Mesh UV mapping (& Probuilder)

Hello! I'm trying to understand how to modify the UVs of a skinned mesh in a straight-forward way. I have a prefab from the asset store that needs the UV mapping to be redone. Only uv1 is used.

I have tried using probuilder, to probuilderize a copy of the mesh and then copy the uvs of t$$anonymous$$s new mesh over to the skinned mesh, but for some reason, when I probuilderize a skinned mesh, it changes the amount of vertices and messes with the mesh in a way that makes it impossible to copy over.

I'd like to understand why it does that and if there's a way around it.