Rotom changes forms in black2/white2 by, going to shopping mall 9, going into the room with boxes and pressing the A button on any one, from there you make rotom forget a move after selecting an appliance and, it changes form!
They can be changed in the shopping mall nine in a room on the bottom floor at the right by talking to the boxes. Source: http://bulbapedia.bulbagarden.net/wiki/Shopping_Mall_Nine
