Zeichen in Textdatei ersetzen

[japetos]

Das ist mein Code:

Delphi-Quellcode:

function file2string(filename:string):string;  
var f: file of char;
    c:char;  
begin  
   result := '';  
   if fileexists(filename) then  
   begin  
     assignfile(f,filename);  
     reset(f);  
     while not eof(f) do  
     begin  
       read (f,c);  
       result := result + c;  
     end;  
     closefile(f);  
   end;  
end;  

 
function strreplace(s,what, whit:string; var count:integer):string;  
var lastfound: integer;  
begin  
   count := 0;  
   result := s;  
   if (what = '') or (whit = '') then exit;  
   lastfound := 0;  
   while pos(uppercase(what), uppercase(copy(result,lastfound+1,length(result)))) > 0 do  
   begin  
     inc(count);  
     lastfound := lastfound + pos(uppercase(what), uppercase(copy(result,lastfound+1,length(result))));  
     result := copy(result,1,lastfound-1) + whit + copy(result,lastfound + length(what),length(result));  
     lastfound := lastfound - (length(what)-length(whit));  
   end;  
end;  

function string2file(filename, aString: string; appendmode:boolean):boolean;  
var f: textfile;//file of char;  
    i : integer;  
begin  
   assignfile(f,filename);  
   if appendmode then append(f) else rewrite(f);  
   for i := 1 to length(aString) do  
     write(f,astring[i]);  
   closefile(f);  
   result := true;  
end;


procedure TForm1.okButtonClick(Sender: TObject);
var  
 dateiname: string;  
 mystring: string;  
 c: integer;  
begin
   dateiname := 'gbuch.txt';
   mystring := strreplace(mystring,'@','[at]',c);
   string2file('gbuch.txt', myString, false);
   showmessage('es wurden ' + inttostr(c) + ' einträge ersetzt');
   //IdFTP1.Put(dateiname, dateiname);
end;

Leider werden die Zeichen nicht ersetzt, sondern es wird alles geloescht.
Kann mir jemand helfen?