Private Sub Command1_Click()
Dim a, b As Long
a = 2
b = 1
Call KK(a, b)
End Sub

Private Sub KK(ByRef P As Long, ByRef Q As Long)
Dim i As Long
If P > Q Then
i = Q
Q = P
P = i
End If
End Sub
引数の方が一致しないと言われます。