The 3520 uses a different chipset than the 3500. According to the review I read, it's a departure from what was before and the 3520 represents a sort of evolutionary new step. Something about the 3500 not being able to be flashed to a 3520A but when the next NEC burner is brought out, if it uses this same chipset as the 3520, it may well be possible to flash the 3520A upwards and have it reflect the same attributes as the new drive.
Read this to become familiar with your drive:
NEC ND-3520A - Modified vs. stock firmware - Introduction
Read this to become more familliar with the firmware:
LD3520 Beta 3 (Liggy & Dee NEC ND-3520A firmware)