Yes you can, all that is needed to change springs is to remove the cap.
You could cut your springs to increase the stiffness of them, which is very easy to do, remember you will need new longer spacers, which most on here like PVC, I personally use black steel pipe, to each his own.
here's some homework for you,
http://faq.ninja250.org/wiki/Front_suspension_upgrades
http://www.ex-500.com/wiki/index.php...the_Suspension in this link pay close attention to the
Cutting Stock Springs ( The FOG suspension Mod ) paragraph.
I personally have done the cut OEM spring modification in both the Ninjette, and EX500 and I'm very pleased with both.