Swap 7-speed for 9-speed I have a Giant Defy 9-speed road bike, and a thief stole the front wheel. I found a good deal on a set of used Bontrager (Edit: Mavic Aksium) wheels, so I bought them as a set. I want to swap both wheels so that they match, and also because the new wheel is 72g lighter. Probably a higher quality wheel overall. The trouble is, it's a 7-speed.

I know how to change a cassette, but I've never changed a hub. I can see that the 7-speed hub is smaller, so 9 gears won't fit. Can I move the 9-speed cassette over? I'm guessing there might be some kind of compatibility issues, but I don't know what to look for. 

Edit: I just realized the Mavic hub has a spacer behind the cassette, which can be removed, leaving enough room for 9 sprockets. But now the locknut doesn't quite tighten all the way, it looks like I need to add a spacer of less than 1mm.
This is why you do the research before buying (and why wheels get stolen). 

If you really want to use the 7-speed wheel, you can leave out one cog from the cassette and use derailleur limit screws to prevent it from shifting outside the cassette.