||11-28-2012 12:55 PM
There's an API for this, you have to edit some magic numbers in a crazy text file hidden in your LoL program folder. Or, more likely, just download something like LoL Item Changer (an external program), have it download all current builds, change them, save them, then export them so they show up in game.
Also, the new shop update will change all of the builds, recommend more and different items, and break those existing build-changing programs and any saved builds you have once it goes live. But I'm sure new programs will be developed some time after the new shop goes live, though it will likely be strange for a while with S3 and all the new items.