'Replace für ältere VB-Versionen
Public Function StringReplace(sText As String, _
  sLookFor As String, sReplaceWith As String) As String
 
  Dim sNew As String
  Dim i As Integer
 
  For i = 1 To Len(sText)
    If Mid$(sText, i, Len(sLookFor)) = sLookFor Then
      sNew = sNew & sReplaceWith
      i = i - 1 + Len(sLookFor)
    Else
      sNew = sNew & Mid$(sText, i, 1)
    End If
  Next
  StringReplace = sNew
End Function
 
TextBox1.Text = StringReplace(TextBox1.Text, "0", "a")
TextBox1.Text = StringReplace(TextBox1.Text, "1", "b")
TextBox1.Text = StringReplace(TextBox1.Text, "2", "c")
TextBox1.Text = StringReplace(TextBox1.Text, "3", "d")