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procedure TForm1.ChangeStrings
          (pStrings1,pStrings2 :TStrings);
var CB :TStrings;
    i  :LongWord;
begin
  CB :=TStrings.Create;
  for i :=0 to (pStrings1.Count-1) do
    CB.Add(pStrings1.Strings[i]);  // Abstrakter Fehler
  pStrings1.Clear;
  for i :=0 to (pStrings2.Count-1) do
    pStrings1.Add(pStrings2.Strings[i]);
  pStrings2.Clear;
  for i :=0 to (CB.Count-1) do
    pStrings2.Add(CB.Strings[i]);
  CB.Free;
end;

procedure TForm1.Button1Click
          (Sender :TObject);
begin
  Self.ChangeStrings(Self.Memo1.Lines,Self.Memo2.Lines);
end;					

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var
    Tmp: TStringList;
begin
    Tmp := TStringList.Create;
    try
        Tmp.Assign(SL1); //1
        SL1.Assign(SL2); //2
        SL2.Assign(Tmp); //3
    finally
        FreeAndNil(Tmp);
    end;
end;