Join Date: Dec 2007
Location: Malvern, PA (20 miles West of Philly)
Bikes: 1986 Alpine (steel road bike), 2009 Ti Habenero, 2013 Specialized Roubaix
Mentioned: 2 Post(s)
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I am in the same exact boat. From reading sheldon brown's excellent website and doing some research, my understanding is that you will need a new rear derailleur....and cassette....and chain...and shifters. You might need new chainrings (opinions differ, the 6 speed rings might be too thick).
It goes without saying, that you will need a new rear hub (i.e. wheel) and have to stretch your rear triangle from 126 mm to 130mm....not a problem if your frame is steel (like mine) but a MAJOR issue if you have carbon fiber or aluminum. See also sheldon brown's site about this as well.
The cheaper alternative (which I am also considering) is to keep the RD-7400 and go for 8 speed shifters....then you would only need the shifters (about 100 bucks used on Ebay), cassette, and chain.
ps Most likely what I will end up doing, to save money, is to get ULTEGRA RD, brifters, chain and cassette, which more reasonably priced, especially from probikekit.com
feel free to drop me a message if you want to discuss this further