Are switches on discount type located throughout your code? Would adding a new discount type require you to modify several such switches? If so you should look into factoring the switch out. If not, using a switch here should be safe.
If there is a lot of discount specific behavior spread throughout your program, you might want to refactor this like:
p.Discount = DiscountFactory.Create(reader[discountTypeIndex]);
Then the discount object contains all the attributes and methods related to figuring out discounts.
