Sub Demo()
Application.ScreenUpdating = False
Dim StrFnd As String, i As Long
StrFnd = "at_case_id,ca_case_ref,at_created_dt,at_audit_type,at_audit_note,at_user_id"
With ActiveDocument.Range
  With .Find
    .ClearFormatting
    .Replacement.ClearFormatting
    .Format = False
    .Forward = True
    .Wrap = wdFindContinue
    .MatchWildcards = True
    .Text = "^13([a-z]{2}[!_])"
    .Replacement.Text = "\1"
    .Execute Replace:=wdReplaceAll
    .Text = "^13{2,}"
    .Replacement.Text = "^p"
    .Execute Replace:=wdReplaceAll
    .Text = Split(StrFnd, ",")(0) & " "
    .Replacement.Text = ""
    .Execute Replace:=wdReplaceAll
    For i = 1 To UBound(Split(StrFnd, ","))
      .Text = "^13" & Split(StrFnd, ",")(i) & " "
      .Replacement.Text = ","
      .Execute Replace:=wdReplaceAll
    Next
  End With
  .InsertBefore StrFnd & vbCr
End With
Application.ScreenUpdating = True
End Sub