Thank you, this was a straightforward solution, which did the job.
In Unreal Engine 5, there seems to be no FOccluderVertexArray, so when I was upgrading, I needed to pass a simple array in to make it work:
void UMyCodeLibrary::UpdateGrass(ALandscapeProxy* landscape)
{
const TArray<FVector> arr;
landscape->UpdateGrass(arr, true);
}