Sounds like a D7, which probably has a freewheel sprocket set. The P8/9 used a freehub and cassette. If the bike is a D7, it most likely also has the Neos derailleur. It's difficult to upgrade the Neos, so it could determine the largest sprocket you can use. If it's a freewheel, that cassette will not work.
Before making any purchases find out what what the existing system is, as it tends to shift from year to year with Dahons. Thor(
http://www.thorusa.com/) is an excellent resource, and hangs out here as well.
if you're interested in tinkering with bikes, check out
http://www.sheldonbrown.com/home.html.