I am replacing a dishwasher which has a copper waterline. However, the store said I should replace it with a flexible aluminum 6 ft. hose. Is this necessary if the existing waterline has been working fine? Or is it a code requirement?

Copper is just fine for dishwasher supply. Its just easier to use a stainless steel braided flexable supply. If you want to reuse the copper, its OK, doesn't violate any codes. Good luck and let us know how things go.