But it sounds like you may be talking about the overall width of the cassette (from the the first cog to the last). The overall width of a 10-speed is slightly narrower than an 8-speed or 9-speed. Those wheels can take an 8-, 9-, or 10-speed cassette (actually 6- or 7-speed, too). Just make sure you use the Mavic spacer behind any Shimano(-compatible) cassette--the spacer should have come with the wheel.
By the way, if you went with a 10-speed cassette, you'd also need a very thin spacer. You'd also need a different spacer for 7-speed.
2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
Most 10 speed cassettes will fit on the same freehubs that fit 8 speed cassettes. Just make sure you remove the spacer from the wheelset that had the 10 speed cassette on it and place it on the other wheelset that previously had the 8 speed cassette. 10 speed cassettes are actually more narrow than 8 or 9 speed cassettes.