If there are no matches, you won't get the prompt to change it. However:

Sub Macro2()
Dim vFindText As Variant
Dim vReplaceText As Variant
Dim oRng As Range
Dim i As Integer, j As Integer, k As Integer, m As Integer
Dim lAsk As Long
Const strFontName As String = "Times New Roman" 'The font to search

    vFindText = Array(Chr(176), Chr(177))
    vReplaceText = Array(-3920, -3919)
    j = 0: k = 0: m = 0
    For i = 0 To UBound(vFindText)
        Set oRng = ActiveDocument.Range
        With oRng.Find
            .Text = vFindText(i)
            .Font.Name = strFontName
            Do While .Execute
                j = j + 1
                oRng.Select
                lAsk = MsgBox("Replace Symbol", vbYesNoCancel)
                If lAsk = 2 Then GoTo lbl_Exit
                If lAsk = 6 Then
                    k = k + 1
                    oRng.InsertSymbol Font:="Symbol", _
                                      CharacterNumber:=vReplaceText(i), _
                                      Unicode:=True
                Else
                    m = m + 1
                End If
                oRng.Collapse 0
            Loop
        End With
    Next i
lbl_Exit:
    If j = 0 Then MsgBox "There are no matches", vbInformation
    If k > 0 Or m > 0 Then MsgBox k & " substitutions made" & vbCr & _
       m & " substitutions skipped", vbInformation
    Exit Sub
End Sub