Can and old range hood or microwave be replaced with a new one

YES.

To help make the transition easy, make sure the customers knows if the current unit is ducted or ductless, and if it is ducted, which way the ducting is running and the size.

This will help the customer choose a proper replacement.

When replacing a microwave, you will want to consider the height of the microwave compared to the height of the range hood and the customer might need to make height adjustments to insure proper ventilation.