AlphZeta, the MD5SUM for that file is correct. I haven't modified that version of the scope, but in theory, it should be easy. You just have to follow the guide. The firmware that your scope has is currently available on Rigol's website.
The problem that the 2.4 SP1 and hardware 58 people (like me) are having is that the firmware that we need is not available to the public. Any lower firmware does not work correctly with out scopes. That is, until drieg's help.
I highly recommend the USB cable method over the serial port method. I used the serial port method first, but was totally stumped why nothing worked until I realized that the scope had a DCE pinout but on a male port. Just soldered together an adapter and I was on my way. I've never seen a cable like that and I don't know if you can buy such a monstrosity off the shelf. If soldering is not for you, the USB method works equally well, you just have to install more software.