Change it to HAM or FRA if possible. DUS & MUC both use the older A330-200, and won't have the latest seats/entertainment.

HAM has one flight a day on the B777-300ER. FRA has 2 flights a day, one on the B777-300ER and the other on a A340. If going from FRA, make sure you book the B777 flight.

As for changing, it depends on your tickets T&C's. Most tickets should be changeable, although you may have to pay.

Cheers,
Rick