Right now, I am running a 52-39 Crank and a 11-23 cassette. Lately I've been having some trouble on some big hills in the area and I was considering dropping down to maybe a 11-25 or 12-25 cassette. My question is if I need to get a whole new cassette, or if I could simply by new sprockets and and replace them on my current cassette (shimano ultegra 6600). Also, would I need to add links to my chain to compensate for the lager sprocket? (I'm using a short cage derailleur too). I tried using the search function, but I sort of got conflicting answers. I was wondering what everyone here though. Thanks guys