Document.changeMode use parameter with this values only:
ChangeMode.BITMAP: int, Value: 5
ChangeMode.CMYK: int, Value: 3
ChangeMode.GRAYSCALE: int, Value: 1
ChangeMode.INDEXEDCOLOR: int, Value: 6
ChangeMode.LAB: int, Value: 4
ChangeMode.MULTICHANNEL: int, Value: 7
ChangeMode.RGB: int, Value: 2