Join Date: Aug 2005
Location: Pittsburgh, PA
Bikes: '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
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Your chain length must let the bike shift to the big-big combination. It's not a combination you should use but, if you ever use it accidentally, the results can be extremely damaging if the chain is too short. So, yes, you will probably have to lengthen the chain. For that matter, you will likely need a new chain if you install a new cassette of any configuration to avoid skipping.
That said, your short cage rd will probably not let you use the small-small or even small chainring-second smallest cog as it will not wrapup enough chain to keep any tension. However, the concequence of shifting into either combination is just noise, not damage.