In Generation IV, it was Electric/Ghost regardless of its form.
Source: Bulbapedia
Yes, you change him back to his original form.
It's only in Generation V that his type changes with his form.
When it does not possess an appliance, it is Normal Rotom, and is Electric/Ghost
When it possesses a microwave oven it becomes Heat Rotom, and is Electric/Fire
When it possesses a washing machine it becomes Wash Rotom, and is Electric/Water
When it possesses a refrigerator it becomes Frost Rotom, and is Electric/Ice
When it possesses a rotary fan it becomes Fan Rotom, and is Electric/Flying
When it possesses a lawn mower it becomes Mow Rotom, and is Electric/Grass