Originally Posted by
daoswald
"Require", I say... your drivetrain performance will be no worse using the same chain length. However, running a longer than necessary chain length is not optimal. Given that you are going to a smaller cassette, you can get away now with optimizing your chain length in a way that could possibly improve drivetrain performance more than was possible with the old cassette and its longer-chain requirement. In theory you could reduce the chain by four links (two outer, two inner) and possibly improve performance slightly because there will be less slack for the derailleur to take up. The smaller cassette enables you to run a chain length that is less demanding on the derailleur's capacity.
Define "optimal". Chain sizing using the longest chain the RD can take up by removing links until the small/small combo is a very common method. More chain=more spread out wear and less tension on the RD cage spring for any given gear=less drivetrain friction. Sizing using the shortest chain length useable results in a severely stretched cage in the big+big combo as well as an unoptimal cage position in any of the larger cogs when on the large chainring.