Using frm2 as New Form2
  frm2.RTB = RichTextBox_Form1
 
  If frm2.ShowDialog() = DialogResult.OK then
    RichTextBox_Form1.Rtf = frm2.RTB.Rtf
  End If
End Using

Private _RTB as RichTextBox
...
 
Public Property RTB() As RichTextBox
  Get 
    Return _RTB
  End Get
 
  Set (ByVal Value As RichTextBox)
    _RTB = Value
  End Set
End Property

Private sub btnOK_Click(...)
If bolChanges = True Then
  Me.DialogResult = Windows.Forms.DialogResult.OK
  RTB.Rtf = RichTextBox_Form2.Rtf
End If
Me.Close()
End Sub