Your proposed modification description is a little short. You need to be more descriptive for your final submission. Each new completely unique level can count as a significant modification. New music that you create can count as a significant modification. New background sprites that you create can count as a significant modification. Simply pulling and replacing a sprite or a music track with something you did not make will not count as a modification by itself. If you do not properly distinguish what you made and credit work that you are using from somewhere/someone else you will have a zero in the final submission so be sure to properly attribute credit.