No. Since the e-Visa is valid for 3 months from the issued date, you must enter Zambia on any date within this period. Failure to enter within this period renders your e-Visa invalid and thus you will you will have to make a new application.

However, if your travel plans do change, as long as your arrival date is still within the 90 days validity period, you will not have to obtain a new e-Visa.