Join Date: Feb 2007
Location: Mid Willamette Valley, Orygun
Bikes: 86 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds.
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When you lengthen a chain, you do it with an inner & outer link, aka a link SET. ONE link SET is good for a 4T increase on the largest rear cog.
When properly sizing a chain, you should have 1 link SET + any fraction of a link SET extra. Therefore, you may have anywhere between 1.0001- 1.9999 link sets of "slop".
It kind of depends on how large your "fraction" of a link set is...
IF you are getting a new cassette, it may be a good time for a new chain anyway, depending on how much use the current one has.
If you bought the lock ring tool and made/bought a chain whip, you could easily swap them back and forth for different riding conditions. The you could keep your old, properly sized chain, for use with your old cassette, although the slightly longer new chain shouldn't present a problem.