Private Function Reemplazar_Aleatorio(Texto_orig As String, caracter_a_sustituir As String, pre As String, post As String)
Dim temp As String
Randomize
 
For x = 1 To Len(Texto_orig)
    If Mid(Texto_orig, x, 1) = caracter_a_sustituir Then
        temp = temp & pre & Int(Rnd * 6) & post
    Else
        temp = temp & Mid(Texto_orig, x, 1)
    End If
Next
Reemplazar_Aleatorio = temp
End Function

Private Sub Form_Load()
texto_nuevo = Reemplazar_Aleatorio("holax, comox andax", "x", "antes", "despues")
Debug.Print texto_nuevo
End Sub

Option Explicit
 
Private Function RndReplace(ByVal sText As String, ByVal sChar As String, ByVal sFirstWord As String, ByVal sEndWord As String) As String
Dim sActualChar                                             As String * 1
Dim lTextLen                                                As Long
Dim Q                                                       As Long
 
    lTextLen = Len(sText)
    If lTextLen > 2 Then
        Call Randomize(Timer)
        For Q = 1 To lTextLen
            sActualChar = Strings.Mid$(sText, Q, 1)
            RndReplace = RndReplace & IIf((sActualChar = sChar), (sFirstWord & CStr(CInt(Rnd * 6)) & sEndWord), sActualChar)
        Next Q
    End If
End Function
 
Private Sub Form_Load()
    ' ¡Vivan las ranas! :D
    Debug.Print RndReplace("holax, comox andax", "x", "ANTES", "DESPUÉS")
End Sub