replace mesh faces

Hi guys,

this is my first tread but I think you really may help me. I think is an easy problem but I really can't solve it.

I have sorted certain faces from a mesh, modified them and now  I want to substitute the originals with the new ones. Can I subtract the sorted faces from the original mesh to leave a space for the new ones? I don't need to join the meshes. I really have problems with "cull faces" and "delete faces" components.

Thanks a lot 

Lorenzo