Human ears are literally just cartilage with a specific shape. The real hearing organ is actually inside the skull. So for your cat ears to be correctly placed, just change the position in which the hearing organs reside in the skull. This would also change the place in the brain to which the nerves are wired to, but this is trivial, so the cat people are plausible.
On the other hand there are some counter arguments. First and more important, human ears are below the center of the brain, in a place at which the brain is quite thin. If you move them towards the thicker part of the brain, there's not enough space for both of them. This fix this, and in answer to your question, either the skull is enlarged or deformed to give space to these organs, or the brain shape changes and adapts to this organ's shape.
Secondly, forward facing ears are, usually, exclusive to predatorial species. So unless the cat people are genetically modified from humans, they should have evolved from a different animal.