procedure FileReplaceChars(AFilename: String);
const
  GBufSize = 500000;
  //Datei durchgehen und umlaute suchen um neue Dateigröße zu berechnen
  function LGetNewSize(AStream: TStream): Int64;
  var LGiveback: Int64;
      LBytes,
      LCount : Integer;
      LBuffer : String;
  begin
    LGiveback := 0;
    SetLength(LBuffer, GBufSize);

    LBytes := AStream.Read(LBuffer[1], GBufSize);
    if LBytes > 0 then
    begin
      repeat
        for LCount := 1 to LBytes do
          if LBuffer[LCount] in ['ä','ö','ü','Ä','Ö','Ü'] then
            inc(LGiveback, 2)
          else
            inc(LGiveback);
         LBytes := AStream.Read(LBuffer[1], GBufSize);
      until LBytes = 0;
    end;
    result := LGiveback;
  end;

  //Umlaut schreiben
  procedure LWriteToStr(var AStr: String; var APos: Integer; const AToWrite: String);
  begin
    AStr[APos] := AToWrite[1];
    AStr[APos + 1] := AToWrite[2];
    inc(APos, 2);
  end;

  //Umlaute in Buffer ersetzen
  function LReplaceInBuf(var ASrc, ADst: String; ACnt: Integer): Integer;
  var LCount,
      LPosDst: Integer;
  begin
    LPosDst := 1;
    for LCount := 1 to ACnt do
    begin
      case ASrc[LCount] of
        'ä': LWriteToStr(ADst, LPosDst, 'ae');
        'ö': LWriteToStr(ADst, LPosDst, 'oe');
        'ü': LWriteToStr(ADst, LPosDst, 'ue');
        'Ä': LWriteToStr(ADst, LPosDst, 'AE');
        'Ö': LWriteToStr(ADst, LPosDst, 'OE');
        'Ü': LWriteToStr(ADst, LPosDst, 'UE');
        else begin
          ADst[LPosDst] := ASrc[LCount];
          inc(LPosDst);
        end;
      end;
    end;
    result := LPosDst - 1;
  end;
  
  //Aus Stream lesen, links von Position
  function LReadBuf(AStream: TStream; APos: Int64; var ABuffer): Integer;
  var LCnt: Integer;
  begin
    if APos < GBufSize - 1 then
      LCnt := APos + 1
    else
      LCnt := GBufSize;
    AStream.Position := APos - LCnt + 1;
    result := AStream.Read(ABuffer, LCnt);
  end;
  
  //Datei durchgehen und umlaute ersetzen
  procedure LReplace(AStream: TStream; LOldSize: Int64);
  var LBufferDst,
      LBufferSrc : String;
      LBytes : Integer;
      LPosDst,
      LPosSrc : Int64;
  begin
    SetLength(LBufferDst, GBufSize * 2);
    SetLength(LBufferSrc, GBufSize);
    LPosSrc := LOldSize - 1;
    LPosDst := AStream.Size;

    LBytes := LReadBuf(AStream, LPosSrc, LBufferSrc[1]);
    if (LBytes > 0) then
    begin
      repeat
        LPosSrc := LPosSrc - LBytes;

        LBytes := LReplaceInBuf(LBufferSrc, LBufferDst, LBytes);
        LPosDst := LPosDst - LBytes;
        AStream.Position := LPosDst;
        AStream.Write(LBufferDst[1], LBytes);

        LBytes := LReadBuf(AStream, LPosSrc, LBufferSrc[1]);
      until LBytes = 0;
    end;
  end;
var LFile : TFileStream;
    LNewSize,
    LOldSize : Int64;
begin
  if FileExists(AFilename) then
  begin
    LFile := TFileStream.Create(AFilename, fmOpenReadWrite);
    LOldSize := LFile.Size;
    LNewSize := LGetNewSize(LFile);
    if (LOldSize <> LNewSize) then //Nur was machen wenn Umlaute gefunden wurden
    begin
      LFile.Size := LNewSize;
      LReplace(LFile, LOldSize);
    end;
    LFile.Free;
  end;
end;