What is needed to change a set of wheels? + advice RE the cassette size
Those wheels come either with a Campagonolo compatible freehub; or a Shimano/SRAM compatible freehub.

If you buy the one for Shimano/SRAM (as of this writing, out of stock in Wiggle), you'll be able to use your existing cassette. To quote @PeteH:


  "The 9-speed Shimano cassette that you already have, the total height
  of it is the same as the total heights of their their 10- and 11-speed
  cassettes. So any of them will fit onto a wheel, provided as @yivi
  says you have the correct freehub. Therefore you're not obliged to
  change from 9-speed."


Otherwise, you'd need to switch to a Campagnolo 10/11 cassette, and presumably change a few more things in your drivetrain for compatibility sake. You can mix and match (compatible) SRAM and Shimano components, but I think that for the most part Campagnolo is not compatible with them.  

You do need to have the proper tools to do it though. A cassette lockring tool and a chainwhip 




Then, it's as easy as this: