Is it possible to change a vertex (height) after combining meshes?

I can change vertices of a mesh I created, using:

Mesh() mesh = new Mesh();
Vector3[] vertices;

mesh = myGameObject.GetComponent<MeshFilter>().mesh;
vertices = myGameObject.GetComponent<MeshFilter>().mesh.vertices;

vertice[index] += Vector3.up * 0.01f;

mesh.vertices = vertices;
myGameObject.GetComponent<MeshFilter>().mesh = mesh;

This works fine but stops working when I first Combine my meshes and then try to change a Vertex.
Is this not possible anymore after Combining a mesh using CombineInstance?
My combined mesh has a vertex count of 1600, none changes, not even when looping through all Vertices.

You can manipulate single vertices in a combined mesh.
Just don’t try it in a Upate() call but in Update()