Join Date: Feb 2007
Location: Mid Willamette Valley, Orygun
Bikes: 86 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds.
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Any change, IF even possible, is so minimal, it really isn't worth it.
You would theoretically shorten the chain by 3/4".
There's a 25% chance you don't have the necessary "excess" on your current chain, since you can only shorten in 1" increments.
IOW, don't bother.
You'll also have the option of throwing the old cassette back on without having to "re-lengthen" the chain.
IF you were going from a 32-34T cog to a 25, I'd definitely say shorten it.