Hmmmm.
I made a post a day ago, but now it seems that it did not post correctly. I must have hit something else instead of post reply. What I get for typing in the dark.

The Power iMIG 250 currently does not change polarity easily. That will be remedied in the future, but as of now, it is a task that requires removing the main cover and rerouting the cables. As Rambozo said, no polarity change is needed to weld aluminum. The only time you might need to change polarity is with flux core.