The Poltergust 9000 is the upgraded version of the Poltergust 3000.
Return of King Boo.Edit
Before King Boo escaped his painting, E. Gadd decided to build an upgraded model of the Poltergust 3000. He packed it with tons of special features, including the ability to detect and suck up ghosts Luigi can't see. When King Boo escaped his painting, he took E. Gadd and the Poltergust 3000, not knowing that E. Gadd built a second model.
The Final BattleEdit
During the Final Battle, Luigi learned another use of the Poltergust 3000 from Professor E. Gadd. He learned that if you press the A button (Wii remote), you can turn it into a powerful jet/vacuum version of the Poltergust. Luigi used this to his advantage and defeated King Boo, thus saving his brother.