[NumLock]

TMemoryStream нельзя к TStringStream приводить.

Код:

procedure Filter(MemoryStream: TMemoryStream;
  const OldPattern, NewPattern: String);
var
  s: String;
begin
  s:=StringReplace(Copy(PChar(MemoryStream.Memory), 1, MemoryStream.Size),
    OldPattern, NewPattern, [rfReplaceAll, rfIgnoreCase]);
  MemoryStream.Size:=0;
  MemoryStream.Write(s[1], Length(s));
end;