Can I change my name through the usercache file in .minecraft?
I think that you can change your real Minecraft name(not server nick) on the Minecraft official website itself. I shortened my name that way.
I know this is a bit old but i just wanted to say this for anyone that sees this. you can change your username in single player only if you want. to do that do into %appdata% -> .minecraft -> usernamecache and change all of the text that has your username to any username you want (ex: Notch) then do that same thing in usercache. after that you should be good to go BUT it will only work in singleplayer