Can I turn my NEC 3520 into a NEC 2500/2501 with firmware?

For compatibility issues i need to dumb down my NEC drive to something that the equipment supports. The NEC 3520 is 16x and the one i want it to be is 8x, right now i cant even use the drive so 8x is better than 0x

Will flashing the firmware do the trick?

No way - you can’t even crossflash between 3520 and 3500.

It may be easier to hack the software, inserting the new drives identity in place of a similar one that it does recognise.