Interesting. I know plenty of people who have PMed me asking why they can't use 1791. And what I find is they installed either the 2006/2007 or 2008 DVD. If what you're saying is correct... that means by some reason your situation is the only exception I've ever run across.
But as you're moving, you hold you finger on a certain point on the screen. Then the diag-box comes up. Once that's up, you enter "1791" nothing actually happens then, you just hit the back arrow, and wallah! you can now change your destination while on the go.
What should happen if the 2006/2007 & 2008 DVD were ever installed, it was known to patch the 1791 trick. Even people on Disco3 forums posted on this. If I'm not mistaken, it will still allow you to pull up the diag box, but when "1791" is entered, destination on the screen is still grayed out and you cannot change your destination on the go.
The only thing I can think of is, maybe ONLY the 2006/2007 DVD installs the 1791 patch. Perhaps the 2008 DVD loading.kwi file assumes that 1791 has already been patched and installs other updates?
Can anybody verify? I'm sure as hell not putting my Land Rover Issued 2006 or 2008 DVD in my car to try. Haha. Someone that thinks they installed the 1791 patch to block it, try doing what "vikjts" did.
Check the following for the Disco3. See the links listed inside as well.
http://www.disco3.co.uk/forum/topic11999.html?highlight=1791