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.
+1 to what Clownbike said. You need to determine if it is a cassette or a freewheel. I assumed cassette 'cause that is what you said. If it is a freewheel you will probably be limited to a 12T or 13T as the highest gear.
I know the Neos will handle a 30T as I have one on my bike. Lower gearing is easy though, just go to a smaller chainring up front.