The Nissan Path

Does a new or used ecm have to be flashed ? If the used ecm is basically from the same equipped vehicle and year, both with same transmission ?

ECM's on WD21's were not flashable ECM's. Nissan didn't start using them until around the year 2000. You need the correct ECM part number for your vehicle. One thing you forgot is emissions certification (Federal vs. California emissions) which will often make a difference.