Late but better than nothing I guess.

Sub Convert2DTo1D()
        Dim arr2D(1 To 2, 1 To 3) As Variant
        Dim arr1D() As Variant
        Dim i As Long, j As Long, k As Long
        ' Populate the 2D array
        arr2D(1, 1) = 1    
        arr2D(1, 2) = 2    
        arr2D(1, 3) = 3    
        arr2D(2, 1) = 4    
        arr2D(2, 2) = 5    
        arr2D(2, 3) = 6
        ' Determine the size of the 1D array
        ReDim arr1D(1 To UBound(arr2D, 1) * UBound(arr2D, 2))
        ' Populate the 1D 
        array    k = 1
        For i = 1 To UBound(arr2D, 1)
                For j = 1 To UBound(arr2D, 2)
                    arr1D(k) = arr2D(i, j)
                    k = k + 1
                Next j
         Next i
         ' Print the 1D array to the Immediate Window
        For i = 1 To UBound(arr1D)
             Debug.Print arr1D(i)
        Next i
End Sub