Quote Originally Posted by bcipam
I switch out my 12-27 for a 12-34 (I put a XTR cassette and derailleur on the rear) and I did need a new chain - the original was just not long enough. If you keep the longer chain on when switching to the 27, then it will be too long and you will have chain slack. There really is a big difference between a 27 and a 34 not so much if you are switching out a 27 for a 25 or 23. ... Also you will need a different derailleur for the 34. The regular Ultegra or whatever you run just isn't long enough. You need a mountain bike or touring derailleur.

Note: I put the XTR on3 years ago for my Utah trip but never replaced it with the 27. Turns out I like having the 34. Are you sure you will be changing the casettes out?...
On this particular bike, I have an XT dearilleur, longer chain, and 11-34. But, its not clear the chain will have too much slack when I move to the 12-27, since it seems to me that since 12 and 27 are both contained within my 11-34 casette, then the slack should be the same when in those gears. But, I'll see when I go to do it. I know I don' tlike the 11-34 for everyday use, cuz the gaps between gears is too large for fast club riding, etc., BUT, if I go to the real mountains or tour I will want it. I am also curious to see how I like the XT rear dearailleur with the 12-27. If its OK, when my racing bike needs a new rear derailleur I may put an XT or XTR on instead of the ultegra I have, again with the idea that I can easily swap casettes ore wheels as needed for the terrain, since I only have a compact double, and not a triple on both bikes.