Almost a year ago I decide that I needed to get back on a bike after about 4 years away. At the time I needed the extra gearing of a triple, but 2000 miles later I'm about 25lbs lighter, much more fit and now it's become just a crutch (I hate climbing and if I've got an extra gear, I tend to grab it even if I don't really need it.) After studying the gear ratios, a 53-39 (standard) crankset combined with my existing 12-25 cassette would impose the discipline I lack.
The question is what all will I need to replace (aside from the crankset itself and assuming I stick with the same bottom bracket type)? I vaguely recall being once told that I should replace the chain when replacing the chainrings. I've also heard that it's a good idea to replace the chain when changing the cassette. From this I gather the rule is: when you change your gears, you should change your chain. Does it work the other way around (change the gears when changing the chain)? How about my dérailleurs? Will the long cage rear be finicky or otherwise annoying (aesthetics aside)?