if filepos(f) > Length(workBuf) then
    seek(f, filepos(f)-Length(workBuf));

procedure TForm1.PatchFile(const path, searchFor, replaceWith:string);
var
r,i,j:integer;
f: file of byte;
offset:array of integer;
buf:array [0..1023] of byte;
workBuf:array of byte;
begin
ProgressBar.Max := GetFileSizeEx(path);
ProgressBar.Position := 0;

//searchFor -> workbuf
SetLength(workBuf, Length(searchFor));
for i:=0 to Length(workBuf)-1 do
  workBuf[i]:=Ord(searchFor[i+1]);

//Durchsuchen
assignfile(f,path);
reset(f);

repeat
  if filepos(f) > Length(workBuf) then
    seek(f, filepos(f)-Length(workBuf));

  blockread(f,buf,Length(buf),r);
  ProgressBar.StepBy(Length(buf));
  //buf nach workBuf durchsuchen
  for i:=0 to Length(buf)-1 do //buf durchgehen
  begin
    if i >= r then break; //Ausserhalb des Buffers abbrechen

    for j := 0 to Length(workBuf)-1 do //Suchstring bei i durchgehen
    begin
      if buf[i+j] = workBuf[j] then //Anfang gefunden!
      begin
        if j = Length(workBuf)-1 then //Komplett gefunden!
        begin
          SetLength(offset,Length(offset)+1);
          offset[Length(offset)-1] := filepos(f) - r + i;
          //offset[n] gibt die Position des workBuf[0] wieder
        end;
      end else //Nicht kmoplett gefgunden!
      begin
        break;
      end;
    end;// for
  end;
until r < Length(Buf);

//Alles was gefunden wurde steht in offset...
//Gefundenes ersetztn:

//replaceWith -> als Byte -> workBuf
SetLength(workBuf,Length(replaceWith)+1);
for i:=0 to Length(workBuf)-1 do
  workBuf[i] := Ord(replaceWith[i+1]);

//An jedem Offset ersetzen:
for i:=0 to Length(offset)-1 do
begin
  Seek(f, offset[i]);
  BlockWrite(f, workBuf[0], Length(workBuf),r);
end;
      
closefile(f);
end;