Kite for long enough and you will try all sorts of silly stuff...
It can be done, but depends on how your lines attach at the bar end, as most bars try to minimise snag points (good) but that means you might have trouble finding a decent spot to connect the other end of the lines to.
Back in the bad old days some line sets came with sleeving that was floating on the line so that when you made the loops at the kite end you had sleeving there instead of bare flying line (which could wear). IF you are keen you should be able to slide sleeving over the existing line ends, and that's probably a good idea if you plan on doing this more than once or twice.
The trickiest part is hooking up the kite, as you need to set the kite end loops in the middle of the (now halved lines) so that they are equal. Best bet is to get someone to hold the bar while you pull each line tight before connecting it to make sure you get it right.
Short lines are fun, but work best on mid -> smaller kites. Get a friend to help you rig, check the setup and launch.
You will normally be able to ride 1 or 2 sizes larger kite due to the smaller power window of shorter lines, but remember that kite will still be able to generate the same amount of power it would have on full length lines to kick your butt, even if it is for a faster / shorter butt kicking than usual. [}:)]
Enjoy!
I nearly stuffed my first overseas kite holiday because of that bloody thing. It would pull like the devil damned but would never complete the loop and I always would land with such force, bloody nearly took my ankle out just before said holiday, so here's me limping around for ages with a brace on. 
