How about:
Private Sub Expand_MouseHover(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button.MouseHover
Dim toolTip As New ToolTip()
toolTip.SetToolTip(Me.Button, "")
If Not Panel1.Visible = True Then
toolTip.SetToolTip(Me.Button, "Expand")
Else
toolTip.SetToolTip(Me.Button, "Collapse")
End If
End Sub