Rotom, Electric/Ghost type, has 5 different form if you get an event Secret Key, see this for the form move
if you have your Rotom to Fan Form (Air Slash) and you want to put it to Frost Form (Blizzard), then Air Slash will be overwritten by Blizzard. and if you want to go back to Base Form, IT'S GONE...and the last move will be empty! it's always overwrite your Rotom last move, so don't put any TM or move in 4th move, or it will be gone forever... that's all that I know, sorry but I'm not sure for the last sentence!
EDIT: If you change your Rotom form for the first time, you can choose a move that you want to overwriten. after that, you can't choose anymore...old move will overwriten by new move automatically!